Fairly straight forward to setup and I am sure google might even find you a setup guide in a bit more detail. Before you begin make sure you have the latest Nexus Setup Utility installed, plug in your usb to mixer to computer.
I have my my CDJ's on channels 2/3 and traktor providing audio on channels 1/4, but you can set your decks on any channels you want.
Once you have decided which channels you want to reserve for you CDJ's. Fire up Traktor, in the setup wizard, select external controller X1, external mixer Nexus and let it complete the wizard.
Once it finishes open up your preferences pane in Traktor. Click on output routing tab and select what channels you want your A,B,C or D on the Nexus to go 0,1 for L/R Channel 1 or 6,7 for L/R Channel 4 etc...
Once the above is configured, select USB as your input on the nexus for the channels you have assigned Traktor to and you should be up and running.