Reply to Problems with DJ io and Traktor Scratch Pro

Hi guys, first post here~
I use a Numark mixtrack and DJ io, connected to the Traktor Scratch Pro software on my macbook. The problem is I can't get the audio right so that I can cue a track on say Deck B, before mixing it with Deck A. I have two sets of speakers and a pair of headphones at my disposal.

Any advice would be much appreciated Thanks!