Hey, thanks a lot for your interest in Pix64! It is actually a tool I initially created to let my kids have fun 'programming' games ;)
Unfortunately building for MacOS is a rather difficult, and obscure task. It is not possible to start the application by double clicking the icon, it must be started from the terminal. Did you tried that?
Open terminal application, then (first line by be different):
cd ~/Downloads/Pix64_1_1/
open -a Pix64.app
(the -a option is needed otherwise it will not find the carts)
if it doesn't work, could you try to type 'mono' in the terminal? The application requires mono to be installed and I'm not sure if it is by default.
If the issue comes from mono I could re-bundle the application in a way that doesn't need it.