One of two ways to go about it (but first make sure you don't have the ability to reassign inputs, otherwise just reassign cd digital input to dvd):
1) Cheap way: use cd or cab/sat coax input for dvd;
2) slightly more expensive, but still cheap: go to Radioshack and get a coax-optical converter (part no. 15-1228) if they still have them. About $25, or there are others out there that do the same (i.e run toslink in and coax out, or vice versa, depending on what you set it for).
Ted_B