M0 PRO native usb port not found when running Zero bootloader (Mac OS)

I just received an Arduino M0 PRO. I wanted to burn the Zero bootloader since it allows programming with Bossac - just like this tutorial uploading Micropython: Load Firmware | MicroPython for SAMD21 | Adafruit Learning System.
My goal is to write the firmware in C and maybe with ASF.

The problem is that after I burned the Zero bootloader, the device is not recognised when plugged in "native usb". It is recognised when in "programming usb". Im running OSX 10.12.6 Arduino 1.8.2 and ARM plugin 1.6.16. There is no "cu" or "tty" device either in Arduino IDE or when searching in terminal.
As a sidenote, I can't burn back to the original bootloader anymore.

Anybody who knows the solution to this? Basically I want to be able to write C code and burn via Bossac or similar terminal software.

Best Michael.