After downloading and installing the mapping into Traktor you will only need to select 900 Nexus In as you cannot set a midi out single with the Nexus. Make sure the midi ON button is lit which is located on the top right corner of the 900 Nexus above the Effects display screen. 

I'll have a picture describing what the features of the Nexus does soon, but your two shift buttons are the LINK CUE button and the FX Cue Button on the mixer. Once on, your Color FX knobs will control either Tempo or Key inside traktor. Be careful to not have filter or another color effect turned on when using it unless intentional. 

LINK CUE - Shift for Key: Deck 1 & 2 color knob controls the key adjust, this is so these knobs can still be used for color effects. 
FX CUE - Shift for Tempo: Deck 3 & 4 color knob controls the tempo adjust for Deck A or B (3 is A - 4 is B), this keeps Deck 1 & 2 free for using the filter or other color FX's when using these two decks for tempo adjust. 

FADER START BUTTONS: The fader start buttons are mapped to load the selected track for deck A & B only. Do note! The 900 Nexus does not have a gate feature so buttons are always set to toggle mode. In this case, when you have loaded a track, the Fader Start button will be lit, you can hit it once to turn the light off without any issues to get ready to load the next track in that deck when ready. I call this the "Double Tap." It was wonky at first, but I now find it as a good visual to know I have a live track loaded since I use Traktor with "Load into Stop Decks Only" Disabled. 

This is the second mapping of my series of 4 different mappings I have developed for the Pioneer DJM 900 Nexus that will really unlike its usability with midi and give you some fun and useful options for both controller and DVS users can add to their live sets and performances.