NAV 2 is tuned via the GTN 650. Due to limited screen real estate, you switch between the COM and NAV tuners by pressing the inner knob of the dual concentric rotary encoder, as indicated by the small "Push for NAV" text on the screen.
There are no big secrets with the autopilot, but I am happy to answer questions. Pressing APPR will arm the glideslope capture, but pressing ATT disarm it and enter attitude hold mode.