If you just purchased the car then you did not do your homework properly
one step is to open the trunk roll the rubber mat back and see if there is no rust on the trunk floor. Just saying
As for your key to the trunk, (as
jpsenft mentioned), do you have the right key? How many keys do you have … I have 4 keys
Ignition … no markings needed it is the largest key.
Doors ……. Stamped "D"
Trunk ……. Stamped "T"
Gas Cap … Stamped "G"
Keys can be tricky, for example when I picked up my car (one item I did not check was all the locks and keys since the previous owner only gave me the ignition key for the test run). When I picked up the car I needed to tank and low and behold I tried to open the gas cap without success. I finally had to phone the previous owner, he came and gave me a lesson how to unlock the gas cap.
So as I said it can be tricky.
If you have no success, I recommend you go to a locksmith or a Benz Dealer, find a seasoned Mechanic and you should be okay then.