Hello
I am guessing that you are using Origin?
If the uninstaller will not work try this - go into Applications on your Mac and look in the folder called “The Sims 3”. This is the location from which your game actually launches (as opposed to a similarly named folder within Documents, which is where your saves and other data are stored). In the folder in Applications you will see the shortcut icons for the base game as well as any expansions or stuff packs installed. If you can see the icon for the game that has failed to properly download from Origin, try right clicking on it and this should give you the option to show the original file. You can move the original files to the trash from there. This may allow you to commence a re-download from Origin. (Many thanks go to Tremayne from TS3 Mac forum for this suggestion.)
Also make sure you trash the Island Paradise-specific trains gaming files to ensure complete uninstallation (but be careful not to trash the other ones!):
http://bluebellflora.com/the-sims-3-on-a-mac/how-to-do-a-clean-uninstall-and-reinstall-on-a-mac/Caspin
