Registration Code for CC 4 Tiberian Twilight

I bought the Ultimate CC 17 game collection and have been playing a few of the games. I went to play CC 4 and it asked for the registration code. When I entered the code from the collection case, it says the code is already used. I've searched my e-mail and cant find any registration code content having been mailed to me. Help if you know what the problem is or have any suggestions. Merry Christmas!