You should be able to use the CD-key. Right click on it and see if it brings up a menu that allows you to input the key and register.
That feature is no longer available. As I said, people who haven't registered retail copies yet must now go to Stardock directly.
Source: http://forums.impulsedriven.com/414366
Nope it won't allow to me right click it or anything. It wont let you register the game if it was bought outside the gamestop app(pure moronic imo) So i'll just email them and see if i get a response... hopefully this doesn't take long or this game is going in the trash.
I'm sorry, but you picked a bad time to try and register. Stardock will have a new App soon that will handle registrations but in the mean time its kind of in a flux. If you would have registered when you got the game there wouldn't be any problem. That said, I'm sure support will straighten it out, though I'm unsure of their hours.