In case the solution in the "Help Guide & Tips" doesn't work for your MacOS, follow these steps:
1. Unzip “PlinkoPlus.app” from the “Mac.zip” to your desktop (so without the “Mac” folder, only “PlinkoPlus.app”)
2. Open “Terminal” on your Mac (on the top bar you chose Go > Utilities and you will find “Terminal”)
3. Type: cd Desktop
4. Type: chmod +x PlinkoPlus.app/Contents/MacOS/‘PlinkoPlus’ (If it doesn’t let you, use sudo chmod +x PlinkoPlus.app/Contents/MacOS/‘PlinkoPlus’)
5. That should be it!
In case you need more help, check this video (also, it's the same situation for other itch.io game):
It goes step by step through all process. Just change with PlinkoPlus.app and ‘PlinkoPlus’