Common problem, especially if they haven't been used much.

For more info go here
http://www.peachparts.com/shopforum/showthread.php?t=141204

Condensed version:
There is a spring loaded pin in the door jam. When the door is closed it pushes the pin into the lock mechanism. The real potential problem is the cable in the wheel well that connects to the foot paw that is held down when the seat cushion (for your butt, not your back) is in the seating position. It's an interlocking safety mechanism for both the seat cushion and seat back.

Assuming door jam pin operates correctly you have two choices:
a] Take it to MB dealer to release or

1b] Flip seat cushion forward
2b] Lay on your back in cargo area, place heels of both feet against top of seat,
retract knees to chest, and gorilla kick the top of seat back. Yes, it can take that much force. Been there -> done that!

Bob
The levers your are fiddling with are interlocks to the seat cushion [for your butt] but don't affect the interlock mechanism for the seat back.