I had the same problem. I'm guessing you have the latest CDex version (1.50 i think). Well, you need an earlier version (1.40 i think). I found a similiar thread in the Divx forum and that's what it recommended. I downloaded an earlier version and CDex worked fine then.
Thanx!!! You know, I think this is so lame! Isn't the meaning of a new version that it should work better??? I encountered the same problem with FlasKMPEG and some other sound program that the newest version worked like shit...so I should use the old one instead. Weird shit ey...