Disk not found after installing Pets Expansion.

Hello, I recently downloaded the sims 3 to my computer, and followed the installation instructions and everything. I did a test run to make sure it worked and all, which it did. But after that, I downloaded the pets expansions. I'm pretty sure I followed all the instructions and stuff, but for some reason when I went to start the game the launcher said: "No game disk found! Please insert the Sims 3 Pets game disk before proceeding.