Thank you for the suggestion!
However, I never bought Sims 3, and I never made an account on thesims3.com. I got the code (from EA) with no information on what to use it for, except to redeem it at origin.com.
It was probably an error to to send this code to me, presuming I had a sims 3 game already. But for now, I can't find out if it is a code for an expansion pack, or if it actually is a code for a game installation that simply isn't handled correctly. And I can't find any way to tell EA support that they have an error in the registration process.
