Garmin 1000 - can't tune it! Am I just dumb?

OK - this is driving me crazy! I was taking my first trip in the Garmin-enhanced Cessna 172 (comes stock with FSX, the glass flight display model)

So I tried to start playing with the GPS - the buttons (PROC, etc...) work like the Shift-3 GPS does in, say, the 737, but I can't find any way to change entries! In other words, the "tuning knob" I would turn to move the highlighted entry up or down before hitting "Enter" doesn't work!

There is a knob at the bottom of the GPS with + and - mouse cursors when you move over it, but it does nothing! Couldn't find a key shortcut for GPS operations. I'm going out of my mind! Is this something MS just left off the panel (kind of like some of the autopilot buttons on the Airbus?)

Hoping for salvation...