My serial code is invalid?

Basically, I wanted to download things for Exchange. But it said I had to register my game. So that's what I went to do. But when I had typed in my serial code it said it was invalid! I bought my game online from the official site, so it's not fake or used by someone else. The thing is, it said that the reason why this happened could have been that since I bought my game on Origin, my game is already registered on my account. So I tried logging in to the Sims 3 site with my origin user details, but it said that I wasn't registered and was asked to register. But they still won't let me download things for exchange. PLEASE HELP!!!!!!