Okay, I've got a solution for launching on Mac and it's kinda stupid, but it's the second time today I've seen it work, so try this:
Do not open the .zip file with the standard Archive Utility on the Mac, i.e. don't just double-click it. Right-click it and choose another decompression tool. I used The Unarchiver.
After it unpacks, launch nwjs by right-clicking and choosing Open.
If you aren't set up or don't know how to right-click on a Mac, try ctrl-click. Does the same thing.
UPDATE: And then it launches to just a black screen. Ah well, I tried. [Trash]ed
UPDATE2: Oh wait, if you double-click the black screen, it says, "Please move the Game.app to a different folder." ...Yep, works from Applications folder... Not ideal, but okay.