For some reason I can't copy and paste from the .pdf of the Manual and I can' be a**sed to retype it but I think this is pages 39 and 40 of the Manual:
While on the Navcom page:
* Press small right knob to activate cursor.
* Use large right knob to move cursor over chosen airfield.
* Press small right knob to select airport and press ENT
* Activate cursor again (small right knob) and
* Highlight desired frequency using large right knob. To place frequency in Standby (COM or VLOC) press ENT while it is highlighted.