Here's what you need to do to run Snap on Mac:
1. Download Just the IWAD, which should be called snapgame.ipk3
2. Go to https://zdoom.org/downloads and download the Mac version of GZDoom (the blue one on the left)
3. Copy the included app bundle to your applications directory
4. Open Finder, then press Command+Shift+G
5. Enter ~/Library/Application Support/
6. Create a folder there called gzdoom
7. Move snapgame.pk3 into the gzdoom folder
8. Run GZDoom -- If you are prompted to select from a list of games, click on Snap the Sentinel and press OK
You should now be playing the game. Whenever an update for the game is released, just replace the old snapgame.ipk3 with the new one from Just the IWAD and you should be all set. I would like to note I am not a Mac user myself, so I won't be able to provide any further assistance. Good luck and have fun!