Locked keys in trunk

The back seats fold down. there is a seatbelt-like ******** the side near the rear window posts that you'll find on either side, the quickest one to open will be the side with the smaller section to pull out (I believe it is the driver's side).
OR the plastic key in the glove compartment (valet key) can also be used.

From what i know the only way to enter the trunk is via the key, Ford unfortunately did not think things through when designing this feature. You need a locksmith sorry to say

Just pull the rear seat down and climb right back there! Didn't you get a spare key when you bought the car anyway though??

You can remove the rear seat and reach threw and pull the inside trunk release . if you look in your owners manual it will show you what it looks like.
there is just a couple of bolts that hold the seat in place.
good luck

On the keys, is there a button to release the trunk? A remote button? If so, you can find the wires that feed the lock solenoid and power it up to pop open the trunk. This does require some technical ability. I hope this helps, or helps someone you know open the trunk. Good luck. Also, you could go to the dealer to buy a new key via the VIN. This is usually a lenghty and paperwork intensive procedure. Be sure to call the dealer and find out what you need before you go there. Again, good luck.