I'm on a Mac with a Ion Discover DJ and Cross LE, and I had the same problem. I looked up for days and found this website:
http://www.Apple.com/pro/techniques/aggregateaudio/It's a little different in Snow Leopard since when you open the audio interface manager all you have to do is click + and then select the interfaces you want to use.
Since then everything works flawlessly, still I'm a little disappointed that my deck doesn't include a plug for cue audio, which cost 25€, but hey, it's a 99€ deck, perfect for beginners except the audio configuration part that really pissed me off. There should be a page on the mixvibes website explaining this, not everybody is a sound engineer (and you definitely are not one when you buy a 99€ set...).
Now everything works fine though, and I am delighted with my DJ set.
Hope this helps, enjoy!!