For Katy Perry's edition, this is the link:
http://www.thesims3.com/incentive/CollectorsRedeem.html
For the Limited Edition, use this one:
http://www.thesims3.com/incentive/ShowtimeRedeem
This sites are for getting the extra content on each game. To know which code to enter go to store.origin.com, click on the "user assistance" tab on the top right corner (not sure what it says exactly, my version of the site's in Spanish and reads "atención al usuario"). You'll see an option at the top of this site that says "view purchase history" (or something like that). Click here and enter your order's data. You'll be redirected to a page with a list of your products and any extra content they came with. The code for the extra content doesn't have the option for downloading from Origin, and it's shorter than the code for the games themselves. Enter this code, WITHOUT the dashes (-) on the corresponding link I've provided above. After a while, you'll get to see the items on your purchase history.
I got helped with this by an EA advisor, it's the only way I could've possibly gotten all of that right, seeing as there are no official instructions anywhere! :roll:
Simplicity is not EA's thing, I guess ¬¬