My MB ML350 stick key (metal key) wont unlock the door. The key goes in, it turns but the lock will not open.

LOCKING AND UNLOCKING
You can use the power door lock control or the remote control to lock and unlock your doors and tailgate.See Tailgate Lock.
Power Door Locks
The power door lock control is on the driver and front passenger door panels.

AUnlock.
BLock. Remote Control
You can use the remote control at any time your vehicle is not running.
Unlocking the Vehicle (Two-Stage Unlock)
Press the button to unlock the driver door. Press the button again within three seconds to unlock all doors.The direction indicators flash. The direction indicators flash twice to confirm the change. The unlocking mode applies to the remote control, keyless entry keypad and intelligent access.
Press and hold both the lock and unlock buttons on the remote control for four seconds to disable or enable two-stage unlocking. Disabling two-stage unlocking allows your entire vehicle to unlock with one press of the button.
Intelligent access at the driver door unlocks your entire vehicle when you disable two-stage unlocking.
Locking the Vehicle
Press the button to lock all doors.The direction indicators flash.
Press the button again within three seconds to confirm that all the doors are closed.The doors lock again, the horn sounds and the direction indicators flash if all the doors are closed.
Mislock
If any door is open, or if the hood is open on vehicles with an anti-theft alarm or remote start, the horn sounds twice and the direction indicators do not flash.
Activating Intelligent Access
General Information
You can unlock and lock the vehicle without taking the keys out of your pocket or purse when your intelligent access key is within 3 ft (1 m) of your vehicle. Intelligent access uses a sensor on the back of the door handle for unlocking and a separate sensor on the face of each door handle for locking.
The system does not function if:

• Your vehicle battery has no charge.
• The key battery has no charge.
• The key frequencies are jammed.

Note: The system may not function if the key is close to metal objects or electronic devices, for example keys or a cell phone.
Note: If the system does not function, use the key blade to lock and unlock your vehicle. See Remote Control.
Unlocking Using Intelligent Access
With your intelligent access key within 3 ft (1 m) of your vehicle, touch the unlock sensor on the back of the door handle for a brief period and then pull on the door handle to unlock, being careful to not touch the lock sensor at the same time or pulling the door handle too quickly. The intelligent access system requires a brief delay to authenticate your intelligent access key fob.
Locking Using Intelligent Access
With your intelligent access key within 3 ft (1 m) of your vehicle, touch the outer door handle lock sensor for approximately one second to lock, being careful to not touch the unlock sensor on the back of the door handle at the same time. After locking, you can immediately pull on the door handle to confirm locking occurred without inadvertently unlocking.
Note: Do not use the outside door handle as a roof cargo strap.
At the Electronic Tailgate (If Equipped)
Press the exterior tailgate release button inside of the tailgate handle. The tailgate unlocks and opens. See Electronic Tailgate.
Smart Unlock
This feature helps to prevent you from locking your intelligent access key inside your vehicle's passenger compartment or rear cargo area.
If you leave your key in the ignition, when you open the driver door and lock your vehicle with the power door lock control, the doors lock then unlock.
You can still lock your vehicle with the key in the ignition by:

• Using the manual lock on the inside of the door.
• Locking the driver door with a key.
• Using the keyless entry keypad.
• Using the lock button on the remote control.

Smart Unlocks for Intelligent Access Keys
This feature helps to prevent you from locking your intelligent access key inside your vehicle's passenger compartment or rear cargo area.
When you lock your vehicle using the power door lock control (with the door open, vehicle in park and ignition off), your vehicle searches for an intelligent access key in the passenger compartment after you close the door. If your vehicle finds a key, all of the doors immediately unlock.
In order to override the smart unlock feature and intentionally lock the intelligent access key inside your vehicle, you can lock your vehicle after all doors are closed by using the keyless entry keypad, pressing the lock button on another intelligent access key or touching the locking area on the handle with another intelligent access key in your hand.
When you open one of the front doors and lock your vehicle using the power door lock control, all doors lock then unlock if the ignition is on.
Autolock
Autolock locks all the doors when:

• All doors are closed.
• The ignition is on.
• You shift into any gear putting your vehicle in motion.
• Your vehicle reaches a speed greater than 12 mph (20 km/h).

Autolock repeats when:

• You open then close any door while the ignition is on and your vehicle speed is 9 mph (15 km/h) or lower.
• Your vehicle reaches a speed greater than 12 mph (20 km/h).

Autounlock
Autounlock unlocks all the doors when:

• The ignition is on, all the doors are closed, and your vehicle has been moving at a speed greater than 12 mph (20 km/h).
• Your vehicle comes to a stop and you switch the ignition off or to the accessory position.
• You open the driver door within 10 minutes of switching the ignition off or to accessory.

Note: The doors do not autounlock if you electronically lock your vehicle after you switch the ignition off and before you open the driver door.
Enabling or Disabling Autounlock
You can enable or disable the autounlock feature in the information display or an authorized dealer can do it for you. See General Information.
Illuminated Entry
The interior lamps and select exterior lamps illuminate when you unlock the doors with the remote entry system.
The illuminated entry system turns off the lights if:

• The ignition is on.
• You press the remote control lock button.
• After 25 seconds of illumination.

The dome lamp does not turn on if the control is set to the off position.
The lights do not turn off if:

• You switch them on with the dimmer control.
• Any door is open.

Battery Saver
The battery saver turns off the interior lamps 30 minutes after you switch off the ignition if a door is open and the dome lamp switch is on. It turns off the interior lamps after 10 minutes if the dome lamp switch is off.
Accessory Mode Battery Saver for Intelligent Access Keys (If Equipped)
If you leave the ignition on after leaving your vehicle, it turns off 15 minutes after you close all of the doors.

Could be the central locking motor or linkage that need looking at.
Can you hear the motor move in the driver door? If so the linkage is at fault, if not then it's the motor

1) Key turns but dose not unlock door. This means that the rod that goes to the back of the lock has came off. This also means that is the reason door is not unlocking with the remote as it pops the lock rod inside the door which pushes the lock open.
2) Key does not turn lock at all but works passengers side door. This means that the locks has been changed in the door, but that doesn't sound like the problem as you say the remote doesn't open the drivers side door so more than likely the rod has came off the back of the lock

You most likely have a central locking system that will stay locked until you can unhook the battery. This is sounds like a similar problem I had with my 95 BMW.

possibly need a lock unlock key remote,disarm alarm also.










Tim

Sounds like the rod that goes from the lock to the key came off. There is usually a little plastic clip that holds it together and they break and you get what you are experiencing. You have to take the inside panel off of the door to get inside it.

sounds like the door lach is sticking
use some wd40 spray the lock and let it sit 4 a while
now since you have the inside skin off reach inside the door with 1 hand and try and open the door from the outside and see if the lock bar moves it usualy pushes down i think on a lach
if it goes up and down with no resistance make sure it is contacting the lach if yes make sure the door is unlocked
somtimes the lock is just stuck
after unlock open the door
push down on lock bar attached to lock then pull up on it if that fails to open door
after that all fails unlock door have friend pull handle and then give the door a good kick from inside just remember not the kick the glass

if you have a valet key, stick it it on the passenger side when unlocking door, so it wont alarm while you open the door hope that helps

It is entirely possible that the unlock button on the remote is worn such that it is not making contact. I've seen this happen many times, as the unlock button is pressed so much more than lock. You might try to reprogram the remote first before you look into getting a new remote. The procedure goes like this. Close and lock all doors Key in and out of the ignition 6 times, hazard lights should flash twice Turn key into accessory or on position Unlock and lock doors with drivers switch Press lock or unlock on remote Hazards should flash again to verify recognition turn key off  manually unlock door and then open and shut it.  you should then be able to test remote. 
The key to the procedure is doing step two quickly Hope this helps, if not you may need another remote or see dealer.

You might want to take the door panel off and check the metal rods that connect on the inside of the door to make sure that one of the plastic inserts hasn't broken. If you find that it has broken there are several different ways to fix that. Some use black tape, some use an epoxy. I don't recommend the epoxy due the the fact that that metal rod turns inside the metal bracket that controls the lock.