Arduino Due Sending Keystroke to Pi

Hi! I have an Arduino Due connected to a Raspberry Pi Model 3. (Using USB) I want to use the Keyboard function of the Due to send the character "P" when instructed. (In OMXPlayer on the Pi, 'P' pauses video)

I'm having a little trouble setting this up. First off, when I plug the Due into my Mac computer, my Mac does not type 'P'.
And I'm a little lost as far as which port to use - Programming Port or Native USB?

Any help or advice would be super! Thanks!

I tried some example code taken from the Arduino Website on the Due, with both USB ports. When I run the program, the Due does not write to the keyboard!