1. Log in to http://www.thesims3.com using your Origin account.

2. If you have any games from this series added to the Origin, then on the right side of the website icons from these games should appear.

3. To add a game look for the option "Register a new game", if this option is not visible, use Ctrl + A and look for it again.

4. After adding the game, wait about 30 minutes and log into your Origin account where the added game should appear allowing its download.

5. If this game won't appear you must contact with  the EA support and ask them to add this game to your Origin game list because it did not appear after you activated it on the website.

If anything else arrises as a problem please just submit a ticket