Look for the digital audio out on the back of your DVD player. Then, look for the audio in jack on the back of the receiver that looks the same. Connect those two together using the appropriate cable. If your DVD player only has the red and white RCA jacks, then it can only handle stereo audio, not surround sound.
If there is not an input specifically for DVD players, then there should be an input marked "axillary". If it is not already in use, then you should be able to plug your DVD player into that input.