CaeraBlue, KT is right about Macs hiding the directory from you. I'll leave you to search for where it hides its application data folders.
I went and used Terminal to make all my files/folders visible... and I still don't have the \GameData\Shared\NonPackaged\Worlds folders anywhere. Does anyone know why this might be?
Installed worlds, regardless of whether they are from the store or not, go into the ...\Documents\Electronic Arts\The Sims 3\InstalledWorlds folder, like Michele said. They are not placed in the game data files, but rather the documents folders, like the rest of things installed via the launcher.
Or am I not supposed to be trying to put them in there at all anyway?
Macs are fucking retarded when it comes to gaming.
I know. Bought one anyway. Can't complain about my own choices.