I already have the Sims 3 installed and by mistake bought the Sims 3 plus pets and I meant to just buy the Sims pets expansion pack. There was a little book in it that has two registration codes, one

Many have been having an issue when installing the SIMS2 PETS expansion pack (EP) and Glamour Life EP.  The best way to resolve this is:
1. BACK UP your game folder first (usually: users + your folder + Documents + EA Games).  You want to back up the EA folder.
2. Download (do not install yet) the SIMS2 COMBO UPDATE patch at http://www.macupdate.com/info.php/id/18988
3. If you have already had the issue (which you probably did, otherwise you would not be reading this), delete your SIMS 2 folder from the HD and the EA games folder from the location mentioned above.  Yes, I know, it is painful but you saved what you needed, so stay with me.
4. Reinstall your SIMS 2 game (the base game).  Run it and make sure it works fine.  This will create a new EA Games folder where the old one was.
5. Now install the PETS expansion pack. If you want, start it to make sure it works fine.  All should be OK.
6. Now run the Combo pack. It will scan your HD to find the SIMS folder and what you need.  A window will open allowing you to select the install. Do it and install it.  If you click the arrow next to the installer options you will see there are patches for all the EP's, but they will be grayed except for the one you need.
7. Install your next expansion pack and run the combo patch installer again, now selecting the appropriate pack to patch.  Again, all others will be grayed except for the one you need.
8. Keep doing this with every expansion you install thereafter.  What the patch does is it updates each expansion pack to make sure it works by solving issues that came up after the release and will update everything up to December 2007.
9. After you are done, copy your original saved EA Games folder back into the location you had it.  You will need to hit 'replace' as SIMS will have installed a new EA Games folder in its place.  It is OK, this is all your games saved data.
10. Start SIMS2 and go look for your saved game!