I fixed my "Disk Not Found" Problem...

It was a simple fix, that I should have tried right away but didn't, and maybe others are in the same boat.  We had Sims 3 and Sims 3 Pets already installed and working, but when we installed Sims 3 Supernatural, we kept getting "Disk not Found".

We solved the problem by:  Using the Sims 3 Uninstaller, and uninstalling everything that had to do with Sims.  Then we re-installed Sims 3, and then Supernatural, using the disks. Then, under downloads my daughter clicked on a Sims 3 icon, and at first we got the "Disk not found" message.  She closed that window, went back to the downloads folder, and saw a different icon for Sims 3, clicked on that, and it worked!!!  She has been playing Sims 3 Supernatural since that moment, and has been ecstatic ever since.  She had been very disappointed, because this was the major Christmas gift she was most looking forward to, and until we did the above, we just could not get it to work...

You can refer to this guide everytime you want to install an expansion pack: https://games4theworld.forumotion.com/t11808-brand-new-how-to-install-the-sims-3-on-mac-os-guide-read-it-here-now-new (especially when you want to install Into the Future and other newer expansion packs/stuff packs).