Edit: I managed to compile & run it on a Leonardo board, but I don't have pins 14/15/16, as it is a different board.
What pins do I map it to? Right now I have set it to pins 3 to 8 and all I see is TX/RX constantly orange, nothing happens and the device doesn't show up as a joystick in Windows 10.
On Leonardo or Micro boards only the 8-9-10-11 are usable, so only 4 channels you have and have to modify the source code according. Not just the pins, but watch out for the array sizes and so. Right now i'm working on a solution, where also these boards can have 6 channels working, without any modifications to the source code. In case you are interested, look for the dev branch on github in a few days from now.