weird... does door3 have a different name or sth? (I'm running flashpoint on mac tho 
No, that should be right (and you can see it listed under 'BackDoor' if you check https://nul.sh/misc/flashpoint/ ).
Are you using the MacOS beta? It looks like the latest version is from 2019- I wonder if you need to update the games list somehow to include newer games. BackDoor 3 came out in 2020, so that could be why. I tried searching for some newer games through the MacOS beta version and they haven't been showing up for me.
I don't really know how updating the games list would be done though, sorry.
FYI: Looks like they got the Newgrounds player working for AS3, so you can play the games there now! e.g. https://www.newgrounds.com/portal/view/761480
Ah, I was think of using the Newgrounds Player for running the game, but I'm discovering that the player isn't supported for MacOS. Sorry about that!
The forwarder tool might work though, I haven't given it a try for macs. Hopefully it should work.