This isn't a bug report but just a tip: I couldn't get the game to start on my MacBook (running 10.14 Mojave) even after trying the usual tricks, right clicking + open etc. So I tried running it from the terminal, i.e. by right clicking->Show Package Contents, navigating to the executable in Contents/MacOS/MewnBase and double clicking it. This makes it run just fine (after turning on Run in low resolution.) Hopefully this can help somebody.

Can confirm this works \o/

Works fine in catalina as well!

Brilliant, it works fine for me. At first it was not opening up at all. Thank you for the tip! Now mac users can play XD