Make sure the VCl-100 is installed correctly (it is a class compliant device so no drivers need to be installed so make sure plugged into a working port). Go to the vestex site
www.vestax.com
and download the Traktor .tks file (the 4deckVCl-100 file worked fine for the demo). Launch Traktor and go into the preferences. Under the midi interfaces select the VCl-100 by double clicking in the active box next to it. Now go to the midi setup and at the bottom where it says setting click on load and
select the .tks file that you downloaded. Click apply and now you are ready to go.