Arduino IDE does not list serial ports on Mac

Hi, I have been trying to use an Arduino Nano and a Adafruit Pro Trinket on all three of my Mac’s, all running Sierra version 10.12.6. In all cases I see no useable serial port in the IDE except for the /dev/cu.Bluetooth-Incoming-Port that is of no use in this case. I have been searching the Internet for days but found no working solution. Anyone any ideas?

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