I don't know if this will work, but it couldn't make things any worse. (BTW, the suggestion by another re: checking with dealer to be sure you have the correct code.) Any, as with some personal computers, removing power from radio for a few minutes, may reset cmptr to accept your code. As I said above, I don't know if this will work, but it couldn't hurt. I have a '92 325i, and have had to enter code for my radio; everything worked just as given in the owner's manual.
James in Colorado