Denon 1508 A/V receiver; sound but no picture

Question

I have purchased a Denon 1508 A/V receiver. I can get the audio portion, but I am having trouble with the video. I have my TV connected with HDMI. When I plug in my cable box via HDMI I get an error message displayed (something about HDCP). I also do not get a picture out of my DVD, which is hooked up by RGB cables. I have tried changing the output from the receiver to the TV from HDMI to RGB but I still don't get a picture. Any ideas as to settings or other ways to fix this?