Did you buy it used? I believe the mixtrack pro came with Traktor LE and if you want it to work with VDJ you will need the over-priced full version. Talk to the guy you got it from and have him de-activated his Traktor license and give you the serial so you can activate (I just went through this with my old Vestax-VCI100).
Maybe not Traktor, seems as though the new ones come with Serato DJ intro, which you can download for free but you may have to purchase the license in order to get two decks working, although I'm not sure. it's a place to start though!