I suggest you connect the HD DVR to the HDMI 1 (Cable/Sat) connection on the back of the SC-BTT273 - and connect the SC-BTT273 to the HDTV using the HDMI out. Page 17 of your manual seems to show this type of arrangement.
You may also be able to use your current connection and connect an optical audio cable from the HD DVR to the Optical Audio Input (Cable/Sat) on the rear of the SC-BTT273.
Just got a new panasonic plasma tv and panasonic SC-BTT273 home theatre/bluray system. We just also had HD dvr installed from comcast. In setting it up I think we have a problem with the home theater system because it only has one HDMI output. If we hook cable to TV and bluray to tv (tv has 4 hdmi) then we cant output the sound from cable programs to home theatre speakers only the tv speakers. We do get surround sound when using dvd. With only the one hdmi I dont see a work around to have the cable programming output to the surround sound speakers. It seems to me that what I have to do is get the SC-BTT370 which has two hdmi inputs and one hdmi output. That way I can hook up the cable box to hdmi input on the home theater bluray box and then take another hdmi cable from hdmi output on bluray to the tv. This should work right? I will be able to hear the sound from cable programming throught the surround sound speakers right? That extra hdmi input will also mean I can attach via hdmi my son's video game console too right?