S Harris is correct. On page two you can select the switch that turns the mix on and off.

Here's another trick that I do. I plug the nose wheel into (I believe Aux 5, the right hand dial on the radio), I then mix rudder-->aux 5, with gear switch down to activate the mix. I turn off the trim link so that the rudder trim does not affect the nose steering servo. The dial on the radio is now your nose wheel steering trim!

This works great because you isolate the centering and trim of the rudder and nose wheel, this becomes very useful as your steering cables break in and start to stretch because you can always re-center the nose without affecting the rudder position. Its great because as you taxi the jet out, if its pulling slightly, just tweak the dial slightly until it tracks straight and you're set!