Insert the cartridge, then listen to what it does: It should pull all six cd-s in (all the way), and then start to play the first. If it just 'clicks' through all of them (and doesn't pull them in the whole way), then you probably have the same problem I (and quite a few other people) had.
I'm no expert but there is a sensor (or something) that has to be replaced for the CDC to detect the CDs properly. That's an approx $20 part, and can be replaced in half an hour by a professional. The whole repair shouldn't really be more than $50-60.