Sometimes the download or installation fails. This can be for a variety of factors, including your internet connection dropping out during the download. Sometimes this can result in an awkward situation in which Origin acts as though the game is installed correctly. This means you can’t play the game, but nor will Origin let you re-download it. You cannot easily “uninstall” the game, because of course it isn’t there.
First thing to do is to 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.)
If this does not work, or if the icon is missing from the “The Sims 3” folder in Applications, then you may need to completely uninstall and reinstall.
You can delete and reinstall the Origin client for Mac and make sure it is up-to-date. The latest version can be downloaded from here:
http://www.dm.origin.com/mac/downloadAdditional advice on how to download games (thank you Tremayne for this tip):
When downloading from Origin, do NOT use the first install icon. Move your cursor over the game cover and click on the information "i" and then click on the install button from there.
For some reason, downloads from Origin frequently become corrupted. Using the "i" reduces the chance of your download being corrupted. If you have already been unfortunate and have a corrupted download, then it is important to remove all traces of this before trying again, by doing a thorough, clean uninstallation first, of the game as well as Origin:
http://bluebellflora.com/the-sims-3-on-a-mac/how-to-do-a-clean-uninstall-and-reinstall-on-a-mac/