you should have the hdmi plug from the cable box plugged into one of the four hdmi ports labeled 1-4, and the hdmi cable going from the denon to the tv plugged into the "monitor out" hdmi port.

make sure that is correct and then try turning the knob on the denon to select the proper source.

I would run the auto setup after that. There should be decent directions for that in the manual.

You might need to manually adjust some of the input settings (such as assigning optical/coax audio with a particular input source (if using any copmponent cables), ect.

-David