It should show up on your Origin dashboard. If not, use the option for them to call you. As another person said, it's already used in Origin so it won't work on the website. This is why I go buy physical copies since it is the same price. Much easier especially if you want to redownload in the future.
Community AdminWe have a sticky about how to register your game her:
http://answers.ea.com/t5/The-Sims-3/Information-How-to-Register-The-Sims-3/m-p/387162
When you purchase The Sims 3 or any of its Expansions or Stuff Packs through Origin, you no longer need to manually register your product through The Sims 3’s website. Just make sure that your Origin account details match the details of your Sims 3 account, and Origin will register your products for you.
