Mole-richardson units are generally 3-prong edison. Unless, however, you bought a studio unit which is probably bates, which I assume is what you are referring to. In any case, there are bates to edison adapters.
Make sure you get the right bates though. It depends on the size of your lamp. Chances are you have a smaller unit and it'll be 20 amp bates, but double check. You don't want to buy a 20 amp bates adapter when you really need 60.