Hello, I have the Sims 3 and other expansion packs. After installing and playing the Pets Game, I installed the Late Night expansion pack and it keeps saying disc not found please enter the Pets disc. I have purchased the game from two different stores and it does the same thing.
Maybe the game prefers that you install expansions in the order that they were released. Try uninstalling Sims 3 completely, but make a backup of the saved game folder somewhere in "My Documents" folder. You don't want to accidentally delete your saved games.
Re-install Sims 3 and all your expansion packs in chronological order based on when they are released. Therefore, based what you currently have you should...
1. Install Sims 3
2. Install Late Night
3. Install Pets