you only need either the HDMI or the rca cables, not both. I would use the HDMI cable from the cable box to the TV. Then on the TV remote there is probably an input button. Select HDMI. Make sure to pick the right HDMI 1-4 that you plugged into. It might only show the inputs you have connected. I'm looking at the manual right now. Doesn't look like your remote has a separate input button. It's probably under Q Menu.
I just got my LG LM6200, but when I connect the cable box to the tv, I get nothing. I have the normal white, yellow, red cable, and hdmi cable, and samsung hd dvr cable box. I click "live tv" and it says no signal. I'm confused, annoyed, etc. Someone please help me? I'd really appreciate it.