I searched for several hours to find out how to get my mac to recognize my new Metro board.
According to this guide:
https://learn.adafruit.com/experimenters-guide-for-metro/mac-setup
(the only one I can find) the drivers will be installed automatically.
This was not true in my case, and there was no guidance how to proceed. I ended up having to hunt down the chip manufacturer to get their drivers which do recognize the board:
https://www.silabs.com/developers/usb-to-uart-bridge-vcp-drivers.
As a new Arduino user this process was quite confusing and I think the guide can be much better. Please update it.