I just installed Arduino IDE 1.8.4 on a RPi3 and want to program Arduino via a USB port. The application seems to run fine except - the Tools->Port->"Serial ports" is grayed out. How to I assign/enable a Pi3 USB port for the Arduino IDE?
You don't have to do anything.
If the Arduino USB is detected it should be shown.
You can check
/dev/tty* to see if the Pi has detected the serial device; this is normally
/dev/ttyACM0 with an Arduino.
Many of the cheap Chinese clones had non-standard USB chips, which are not detected, and are notoriously unreliable.
The cheap arduino clones use the CH340G Chip to control the uploading via USB, a different chip than the authentic arduinos. I have a handful of them that spontaneously like to not work with my IDE.
This site might give you a little more insight than I can:
Basically, sometimes you might need to install a 'driver' for your system to be able to communicate with the chip over USB. Once installed, you should be able to see the device in your Tools -> Port options.
Another thing you might need to consider if you didn't know, is there are two different types of Arduino IDE, and I recommend using .cc version if you are not using that already.