your the sims 3 account uses the same email and password as your origin account so just try log in to the sims 3 site with that email and password if you registered the game in your origin account first you do not need to reregister the game code on the sims 3 site if you registered the game in origin first they auto reg for you to your the sims 3 account although sometimes it can take an hour or more and up to 24 hours for the game icon to show up on your my page
read this link for more info http://answers.ea.com/t5/The-Sims-3/Information-How-to-Register-The-Sims-3/m-p/387162#U387162
if after when you log in and it still says you need to register a game even though you did contact CS to see if they can assist
if in the event you need to contact CS
1.https://help.ea.com/en/contact-us/
2.Click on the orange NEXT button
3.Select a platform
4.Click on the orange NEXT button
5.Select a topic and describe your problem in a few words
6.Click on the orange NEXT button
7.Click Chat Now to chat with a Game Advisor
guide on how to contact CS
http://answers.ea.com/t5/Beta-Archive/How-to-contact-a-Game-Advisor-using-the-new-EA-Help-page/td-p/2161281
edit typo