Mirror lockup for the Nikon F6 is purely for avoiding mirror vibration. You turn the mode dial to MUP mode, press the release button once and the mirror goes up, press it another time the shutter is release and then the mirror goes down. 30 seconds after the shutter release is pressed the first time the shutter will goes if you don't press it a second time.
It was not designed for using with old lenses that protrude into the mirror chamber like early F or F2.