Minecraft wiki entry for Migrating_saved_game_data_to_the_cloud, associated text: GUI
Creating symbolic links in MacOS is also easy thanks to a free application called SymbolicLinker, which will allow you to create a symbolic link via right-click in the Finder. If you'd rather use the command line, instructions are below.
Download and install SymbolicLinker.
Once installed, move your Minecraft saves folder (located in '~/Library/Application Support/minecraft) to a folder in your Dropbox.
Right-click on this new folder and choose Make symbolic link. This will create a symbolic link in the same folder.
Move this back to your Minecraft folder's original destination (make sure it has the same name as the original),
and you are all set to pull your game data off the cloud.