How to register to sims 3?

Szaszlykkrewetki​  Go to http://www.thesims3.com , sign in with the email address and password you use for your EA account, and you may be prompted to register your game right then.  If not, or if you have more codes to add, scroll down to the bottom of the page and click Register a Game.  (Redeem a Code is for Store worlds and other bonus content, not expansion or stuff packs.)  You can register all your DLC this way, that is if the codes haven't been used already.

If you ever did register your content on the Sims 3 website, including on the Store part of the site, it should still be there, attached to whatever account you used at the time.