All of your maps are stored locally. Considering you posted
this a few days ago, did you just change your release stream, or did you download one of the other versions on the osx.ppy.sh link I provided?
If you just switched your release stream in the options menu, you may need to hold FN while hitting F5 on the song selection screen. This is because the media keys on most mac keyboards are usually set up to be the primary function of the key.
If you downloaded an entirely different release, you'll need to transfer your maps from your old installation to the new one you just downloaded. This can be done by right clicking on the old version of the game and hitting "Show Package Contents", then going "Contents > Resources > drive_c > osu! > Songs" and copying all of the files in that folder over to the same location in the new installation. Then, once you open the game again you should see all of your maps start to process, but if the don't, try hitting F5 on the song selection screen once again.