Registration

I recently purchased the command and conquer ultimate collection, and was able to download all of the games, using the code, successfully. When I tried to play command and conquer 4 it would ask me to sign in then choose a persona. After that it asked for a registration code. I tried using the one that downloaded all of the games, but when I do it just states that the code has already been used. So is there a place I need to go to get a registration code or any way to get around this?