Hello there, thanks for commenting. Unfortunately, Godot (the engine Pixelorama is made in) does not allow exporting in .dmg format if you're using Windows or Linux. In order to export to .dmg, we would need to do it from a Mac, which we currently don't have.
There's not much we can do right now about it since we don't own a Mac machine, but we'll update the install instructions based on what @rafaelgdp said!