Hi there,
I gave the one I had to @cgwaltney but I know this when I tested with it a minute back 
CircuitPython includes full USB support out-of-the-box — it mounts as a mass storage device (CIRCUITPY).
MicroPython, however, does not automatically do this unless the firmware is specifically built with USB CDC support enabled.
I flashed a vanilla or outdated MicroPython .uf2, it did not include USB serial or was built for a different RP2040 board. if I recall , frustrating the first time but just Use the official UF2 image built for the RP2040. But better: use Adafruit’s RP2040 MicroPython build or a Seeed-custom version if available by now?.YMMV 
Recommended MicroPython UF2 download for Xiao RP2040:
MicroPython - Python for microcontrollers
Click “Generic RP2040” if available
Or build your own with USB CDC enabled.
I don’t do Mac but,
Once flashed, the board may not show up as a storage device (like CircuitPython), but it should show up as a USB serial port.
On macOS, it should appear as something like:/dev/tty.usbmodemXXXXXX
Use a serial terminal like: screen /dev/tty.usbmodemXXXXXX 115200
IF nothing appears, it’s likely flashed with an image with no USB serial support.
HTH
GL 
Pj 