It seems that the printer messed up the Registration and Activation codes on the Sims 3 Plus Pets game, so here is some help from the Sims team regarding it.
Thank you for purchasing The Sims 3 plus Pets.
When you install The Sims 3 base game, please use the code located on the back of your manual above “The Sims 3TM Code”. If you are still having issues installing The Sims 3 base game, please contact Technical Support. You can find their contact information in the game manual.
When you install The Sims 3 Pets Expansion Pack game, please look for the red Pet Store insert in your box and use the code found on the insert. You can also use this code to get your Pet Store. Please read the instructions located on www.TheSims3.com/incentive/PetsRedeem for more information.
The code located on the back of your manual above “The SimsTM 3 Pets Code” should be disregarded.