I'm not sure how long ago this post was made..I too was having the same issue..When you buy the game on Steam you get a Product code to put into your Origin account. after you have signed into your account go to your games and in the top right corner should be a tab that says add a game (Click Redeem Product Code) and then you should be all set, I hope this helps someone
@kidowash420 Do you already have an account in Origin, or maybe the sims 3 store? You need to create one first in order to register your base game and packs in either place; once you've input the codes, the other site will sync the information. If the store isn't working for you, try clearing your browser cache. When using the store, be sure to select "register a game" rather than "redeem a code."
If you're trying to do something else entirely, please describe in more specifically.
@HeartAttackJac1 The store was under maintenance last week, and it seems like things aren't quite back to normal yet. (You're not the only one frustrated around here.) Before you give up, though, try using Firefox. People have had better luck with that browser lately.
If this doesn't help, I'll forward your information to someone from EA. Any other information you can provide—screenshots of error messages, that kind of thing—would be helpful.