Are you pressing the fn button while pressing F5?

You need to do that for the F5 button to work, or else all it won't do anything.
The fn button is located near the top right hand corner in a group of 9 buttons( For my keyboard that is)

Seems interesting, being new to M.L.U.E. myself, this would be a great way to get better at the game.

Oh! Oh! Can i be the guy who brings donuts to every meeting?
Mmmmmmm foood

Odd, it works for me. I'm on a Mac though, so in all fairness it shouldn't work based on how many times in the past my computer has utterly failed.