Re: Can't register downloaded game?
Did you buy the game from Origin?  If you did it is automatically registered for you on the Sims site; it just takes some time to show up on your "my page."  Please read either of these helpful posts to make sure you're following the right steps for registering your games (even if you didn't buy them from Origin):
-- http://answers.ea.com/t5/The-Sims-3/Information-How-to-Register-The-Sims-3/m-p/387162
-- http://forum.thesims3.com/jforum/posts/list/656133.page
-- http://forum.thesims3.com/jforum/posts/list/655809.page
Best of luck