Which device drivers do I need for OSX Sierra + Arduin Uno with Mega16U2 chip?

I can upload code fine to my ATMega chip on a standalone breadboard, using a usb-to-serial device.
So I know the chip is OK.
But when I put the same ATMega chip in my Arduino board, and connect it with a USB cable (one that used to work, so it’s not just a power cable), the board doesn’t appear in System Report in the USB tree anywhere.

I think this is a driver issue, but the Arduino does not have an FTDI chip or a CH34x chip.
It has a Mega16U2 chip.

Do I need the FTDI drivers, or the Silabs drivers, or the build-in OSX driver, or what?