Supposing that you have defined your model as "Airplane", "2 ailerons", "2 flaps", "ailevator"
You'll have channel 10 available as AUX3.
Go to:
Linkage menu->Function->AUX3 ( page 3 of Function )
Rename AUX 3 as "steering" and assign to control LD or RD at your convenience, with no trim.
Now plug your steering servo on channel 10 and check the way and travel in the servo reverse and ATV pages of the linkage menu.
The ATV page shall be setup so that a full deflection of the volume control ( RD or LD ) gives you the full deflection capability of the nose wheel steering.

Then setup your mix as followed:
Go to:
Model menu->prog mixes->MIX1 ( supposing that this line is available )
Then highlight "inhibit" and make it "active". Keep mode as "Mixing" and group as "group" ( unless you want to assign the mixing at a flight condition, which is a very elegant alternative way of doing it buta bit more complex )
On the page 1 of the mixer setup, keep the mixer as linear ( at least initially ) with the rates you need ( maybe +- 60%, so that the steering when operated from the rudder control is less sensitive than when operated from the volume knob. This is called in real aviation the tiller function, where the tiller gives you something like +-70 degrees of travel, whereas the rudder pedals give you only +-10% for example )
Go to page 2 of the mixer setup and assign master as rudder and slave as steering.
Go to page 3 of the mixer setup and assign in "switch" the name of your gear switch. Highlight the ON/OFF function, and select the gear up position as OFF and gear down position as ON...