Problem with serial number for Traktor?

I bought a Numark Mixtrack from my friend yesterday, but he did not have a serial number with it. He says when he bought it, the software did not work and neither did the serial number. So I downloaded Traktor LE from numark.com and I'm having to use the demo. I'm getting tired of having to close out every 30 minutes and lose my work so how could I get a serial number without paying more than I already have?