Open up the door and look behind the petal, do you see this black knob down there? If so, push hard on it and see if it CLICKS back, it should then pop back out. Yours is likely either broken or gone. Buy a new one! You can get it from Phil @ Fastlane, just be sure he looks under Acceleration Control (it's not under transmission in the Mercedes computer, they messed up).
It's the black circular thing below and to the right of the white screw, and it's directly behind the gas petal when floored.
Here is a closer look:
Do you see this on your car??
(BTW, when they put in a new transmission, I wouldn't think they put in a new kickdown switch if it's operating properly...)