I bought C&C4 yesterday, came home installed it and put in the key and it said it was already in use. I went back and exchanged it for another one, reinstalled, and it said that registration key was in use as well. WTF?
The reason it is telling you that the key is already in use is that you already have registered that key with your email address and password and you are trying to register it again. When you got another copy of the game maybe you repeated your mistake. Next time you try it, use the first email address and password that you used and importantly click on the button "Login". Give it a go and let us know what happened. If you still have a problem you can try emailing EA Support, use the "Contact Us" link on the EA webpage at this address. http://support.ea.com/