Installing the silabs Silicon Labs VCP Driver.pkg worked for me also. 👍
(on OS X 10.11.6)
Seems obvious that we need the drivers, but I thought I already had them after going thru the espressif/arduino-esp32 install steps.
e.g. My "System Report / Hardware / USB" showed
CP2102 USB to UART Bridge Controller driver
For others' future reference, you should see something like:
n8s-iMac:nleon$ ls -al /dev/tty.*
crw-rw-rw- 1 root wheel 18, 2 Jul 4 11:14 /dev/tty.SLAB_USBtoUART
Regardless, thanks for the pointer!