The dimming function only works if the light sensor has has turn the head lights on if you have the headlights set to auto. If you turn the headlights to on manually the dimmer wheel will control the intensity. I play with this future quite bit because for my taste the dash lighting are to bright for night driving outside of the city. Specially the blue "D" on the shift lever indication lights.
Actually I did notice that during the day with the heads lights off, The dimmer still dims the gauges. Only the HU responds to head lights on or Off (2 brightness settings but not adjustable using the dimmer wheel). I think you should go back to the dealer.