No serial port appears when Arduino is plugged in

I'm taking over a project from someone else. I've been given an Uno and a Pro Micro that have already been programmed and apparently worked fine. However, when I plug either one into my MacBook Air, my laptop doesn't recognize them - nothing new appears in the list in Arduino => Tools => Ports. I also tried both of them with a friend's PC and the same is true there. Is there something the previous person using these boards might have done that is preventing them from appearing in the serial ports list?

same issue.

some people got it solved following

What USB to serial converter have you got? Post a link to where you got the board from or a picture of the board if you are unsure.