How do i add transmission fluid to my 2006 750Li?

Two litres? Unless you are outside the USA, transmission fluid generally comes in quart containers. If you only added 2 quarts, yes, it's very possible the fluid level is low. However, you don't want to add too much. There should be a dip stick for checking fluid level. Run the vehicle until the engine is good a warm, about 15-20 minutes, and then check the fluid level. Add as needed. But remember, don't add too much. Add a little at a time, as you get close to full.

If you have your owners manual, there is a section containing the capacities for all fluids. You may also simply check your fluid level, with a running motor and warm transmission. If the stick indicates add, start with a 1/4 quart and work your way up. Unless there is a problem with the trans, there is no real reason to ever add fluid unless your are flushing the transmission.

pour the transmission fluid in the tube where the dipstick is located

to add transmission fluid,lift the hood.
look near rear of engine for a thin tube with a dipstick
NOT THE engine oil dipstick,.
to add transmission fluid,you will need a narrow funnel that will fit in dipstick tube.
and that is where you add the fluid

Add your fluid through the tube that your transmission dip sick come out of. Take out your transmission dipstick(make sure it is not the oil dipstick) put a funnel in the tube and add the trans fluid. Check the fluid when the trans is warm because trans fluid expands with heat and you want to make sure you don't overfill. Add small amounts at a time.

You add fluid through the same pipe that the Dip stick for the transmission is located. Remove the transmission dip stick and insert a funnel in that same pipe and add fluid there. Never add more than about a pint of fluid at a time. Unlike the engine where you typically add a quart at a time when it is at the add mark in the Transmission you will add a pint if it is at the Add mark.

TRANSMISSION FILL
To avoid overfilling transmission after a fluid change or overhaul, perform the following procedure:

1. Verify that the vehicle is parked on a level surface.
2. Remove the dipstick tube cap.
3. Add following initial quantity of Mopar ATF +4, Automatic Transmission Fluid, to the transmission:

1. If only fluid and filter were changed, add 5.0 L (10.6 pts.) of transmission fluid to transmission.
2. If the transmission was completely overhauled or the torque converter was replaced or drained, add 7.7 L (16.3 pts.) of transmission fluid to transmission.

1. Check the transmission fluid and adjust as required

If you have a dipstick under your hood for the tranny fluid, then fill it in there slowly... if not, you'll need to fill it through the filler plug that will be on the side of the transmission just above the halfway point

there are two areas where you can add fluid from the front of the vehicle, doesnt matter what kind of car it is, one is for checking the oil the other is for the transmission fluid, but before you add transmission fluid i recommend your flush out the old transmission fluid first so all the grimey old fluid is out and at the same time can notice if any bits of metal are floating around

You add A/T transmission fluid at the same place you check it. 
Same place you check the A/T fluid level. You add fluid at the same location.