The AV receiver/amplifier has to have the ability to route the rear channels through the PRE OUT ports. These are generally labeled Side or Rear Left/Right. You may have to use your AV receiver menu to route the rear signals through these ports and "activate them." For Denon, this will be labeled Connecting an External Power Amplifier. For more information, consult the user's manual of your AV receiver or amplifier.
If you don't have a line-out port on your device, you will need a Level Converter.
This is a device that converts the high-level signal into a line signal.