Which script are you currently using? I've modded the FPSWalker script of days gone by to turn gravity off, if that's any clue. Look in your code where gravity is applied. Make a 'flying' var (boolean) that when it's 'true' you don't apply gravity. And have Update look for the 'f' key to toggle that 'flying' boolean.

var flying = false;

function Update()
{
  if (Input.GetKeyDown("F"))
   flying = !flying;
}

What i want is for the player to be able to togle it on or off.