Depending on how you purchased it...
Retail Disc/Box: It would be found in/on the CD Case/Packaging, or printed on a Manual/Paper Insert inside the case.
Digital Download: It would be in a Purchase/Confirmation E-mail. Don't forget to check your Email spam folder. Also check if you have any Spam Blockers enabled, which could prevent the Email from getting to you.
For more detailed information, here's a great Guide: Sims 3 Store Worlds
