Hi there! I'm very familiar with this very arduous process on MacOS. Here's a quick guide.
Download the base version of the game AND Download the mac-only patch (see attached image)
Download the latest patch & Open it in a New Finder Window
In a new (second) Finder Window, navigate to your Game folder in Pokemon Desolation /Applications/Pokemon Desolation --> Right Click "Show Contents" --> Navigate inside your Game folder. Copy each folder (Audio, Data, Scripts, Graphics) FROM the Patch folder INTO the Game folder, and when prompted to Replace or Merge, click Merge. If you are only seeing "Stop" and "Replace" like in the image below, you need to make sure you Copy/Paste and NOT Drag the folders.
Open Desolation, walk out of whatever map you are currently on and walk back into it to refresh the map with the latest patch. (If you are starting from the beginning, ignore this step)
Enjoy the game!
Disclaimer: selecting "Replace" in step 4 will break your build of Desolation and you will have to re-download the base game to get a fresh copy and repeat this again. Additionally, do not drag the folders (Audio, Data, Scripts, Graphics) or you will not be prompted with the option to Merge. You must copy and paste (cmd c + cmd v shortcuts will work.)
Please let me know if it works! Otherwise, feel free to dm me on discord @dariusmcomposer.