Hate to sound like a jerk but did you replace the batteries. I was in O'Rielly's the other day getting some plugs for the TR6 and noticed they had a key fob tester on the counter. It tested the batteries. Might be worth a shot.

The keys have a micro chip inside that is programmed with your vin. If it came from Ebay or another car the spare you have is useless. It CAN NOT be reprogramed or setup like a new key.

Dealer is best place to get a replacement. Cost is about $130. It comes from Mercedes out of Fort Worth area. You will need your registration/title and a picture ID at most dealers before they will oeder one. Oh you will also need to bring the complete VIN. Make sure it is correct or you bought a useless key. NO EXCHANGES or RETURNS. You prepay for the key.