Help Sims 3 Error Message!!!

Okay, so I was out of the city and I forgot my pets expansion pack disk at home, so I went out and bought the showtime expansion pack and installed it. (I have a MAC) Once I opened up the game it said there were new updates, so I press okay and it says "downloading" and then it says "some of the files have been modified, please reinstall" So I press quit (the only option it gave me) and I reopen the game, when I do it says, again, that there is a soft ware update available, so this time I pressed cancel. I pressed play, the screen goes black, like when you usually start up the game, and then it brings me back to my screen and shows the message, "It looks like you have installed and unofficail game modification that does not match your current version of the sims 3...."
I have tried uninstalling and reinstalling, I have tried to download the fixes, but my computer won't open them.
I've looked at websites and I have had no idea what people where talking about when they used the words "cracks" or "caches"
So please can someone walk me through how to fix this?????