In October 2011, EA released a game update (patch version 1.26 for PC, 1.27 for Mac) to coincide with the release of the Pets EP. These both included substantial changes to the way Custom Content (CC) works, to enable the animations of Pets to work with objects they could interact with.
The result was that all pre-existing CC was broke and failed to work in the updates games.
TSR have worked tirelessly for days to fix the problem for the entire Sims community. First we fixed our Workshop tool so that all newly created CC would work with the latest game versions. Then we started fixing all of our Sims 3 downloads on the server so that newly downloaded content would also work. Finally, and this is why you are here, we released a tool to fix players’ games, patching their downloaded CC as well as that already installed in their games. The result was a "fix it all" tool to get players games working again. It’s called "TSR Pets CC Bug Fixer", and you can download it in step 1 of our guide.
Note that you will need to update your game to at least 1.26 for PC, 1.27 for Mac (or install the Pets EP) for any new CC to work which you download after October 20th 2011.
Unfortunately this tool will not work for Mac users. Fixed sims3packs can be downloaded from the site and these will work on your Mac. You could also copy your Sims3pack files to a PC and fix them all in one batch to drop back onto your Mac, so there are ways to fix your game, but not as quick and easy I'm afraid.