key fob

I have a S500 mercedes 2000,and just bought a used Fob Key, does anyone know haw to program the key Fob (wireless Key)?
Thanks for your help

Dealer only for Mercedes since 1996, but normally they do not charge to reprogram. However, 98+ with the Smart Key (infrared key) has to be turned ON to enter programming mode....so if it's wrong key, it can not be programmed even by the dealership.