I have a Mac—UH, I have a similar issue with the other games of this creator except for this one (first game that let me actually PLAY 💔).
If it keeps telling you ‘is not from a known source’ (or basically from a third party website)—right in that message, there should be a question mark button. From there it tells you that you’re able to allow access either way in settings (don’t worry it explains some instructions from what I remember). It could be a bit different from your view but that’s how I did it 😅. But basically there’s a setting to allow the game to run either way despite it being from a third-party website.
holy essay of mine mb 💔, hope it helps