So, if you export with NW.js, then you should get five folders, one of them being "osx64".
The contents of that folder will be your export.
Now I don't have any experience with MacOS, so I can't directly help you with MacOS.
You might wanna have a look at this, especially at the "Publishing on OS X and Linux" section.
npm install -g nwjs-macappstore-builder
Also, this looks like a command line execution to me.
I believe, on OS X, it's called terminal. You'll have to google the rest for yourself, as I said, I never used OS X.