It should show up on Origin, if not just do the Origin live chat and give the rep your product code that you used to register on The Sims 3 site. Sometimes it takes a while for the two to sync up, but they should be able to manually fix it up for you. A few times when I registered on The Sims 3 site it took a while to show up on Origin but the live chat people were able to help me out.
Hope that helps!
Edit: The Origin people will be able to access your account on The Sims 3 and see that it's registered, so they can add it into the Origin page for you if it hasn't shown up already.