i've had this issue before with the main sims 3 game. I've had mostly online downloads for my installations and recently tried Origin's online beta for Mac. When i installed just the sims 3 base game i received the error unknown message. Even after installing seasons it wouldn't let me get past that error. Here's where my specific actions fixed it; i uninstalled seasons, and started the uninstaller for the base game and left it at the screen where you select which installation to download (basegame, adventures, etc), the next screen would confirm. I started the game with the uninstaller open and it then asked me if i'd like to update, getting me past whatever was causing the error message. The only logical thing i can apprehend from this was the uninstaller being open while starting the base game, or uninstalling an add-on/expansion and then (with or without the uninstaller open i'm not sure) opening the base game. I know this is confusing but try with the uninstaller open but not all the way to the actual process of removing files.