Well it should sit flush. Make sure that the cable is oriented the right side up. They only go in one way.

see when i try to make it sit flush, i feel like i have to force it in so hard that i'm gonna break something. when i look at the end of the cable, it looks like it should snap into place or something, but it won't snap on either the digi or the mackie. does that make you think that it's the cable?

Probably the wrong cable. The end should look similar to a USB-B cable with a non-symetrical hexagon shaped head. Okay, technically it's not a hexagon but it's six sided.

You might well have a TOSLINK cable with 1/8" optical minijack connectors on each end rather than standard Toshiba/Sharp optical connectors. Test this by seeing whether your optical cable will physically fit into the headphone jack on a personal stereo. It shouldn't.

thank you for the replies gentlemen. i finally got it to work. i went to guitar center and bought a new cable: the original one was a really cheap one, so figured that something was wrong with it. when i looked at the good one and looked back at the bad one, the plastic "snap" (for lack of a better word) that holds the cable in place was MUCH bigger than the other one, which surprised me since it was on both ends of the cable. now i have the newer one in place and am getting signal quite nicely.

thanks again fellas,

-dave