Not sure how you had a 6 audio out DVD. The receivers, 5.1 surround, have 6 outputs that connect to your six speakers (L, R, center, LR, RR) but the external audio inputs were always just L and R RCA connectors. How about a link to the receiver? If you are using it as a switcher for your inputs, IE you have video routing through it as well, you might have multiple inputs for that (R G B and sync perhaps). Post the model and link to the receiver.
OK, dude, or should I say old chap, where did you get that "new" DVD? the 500 only has composite audio and video out. Return the thing and get one with an HDMI out at least just in case you get a TV and or Stereo that can use one. Sport for one with WiFi so you can pull up Netfix etc on it as well. I see them for $50 in the US.
Really need to see specs on the receiver to give you definites, but I am guessing you are trying to use the receiver to switch input devices (DVD, VCR, Cable Box, etc). I again have to assume its already connected to your TV and it works so whatever the video format you are passing through the receiver is currently working. If you really do have 6 input ports for each input on that receiver, I am guessing you have at least one input that has a left and right for audio and a Composite Video that will accommodate that 3rd RCA cable. It should work but seriously, in a world of insanely high resolution you are shooting for a step above VHS.