There are not a lot of examples of Arduino communication on Mac platforms, so here you go!
It is nothing fancy, and should build with Xcode 10 or later with no (or few) warnings. This code is based on a serial example written by Gabe Ghearing in 2009; so it was a bit dated and would not compile. I still have a few bugs to workout, but overall it does work.
If you just an Arduino Serial Monitor for Mac, the debug folder should have the app included.