You should be doing it through Impulse. Install Impulse and reboot (you can get Impulse from WC homepage or from stardock.com or Impulsedriven). Reboot.

In the upper right corner of Impulse click on the registrations and then just type in the email you used when ordering (also keep a record of that email).

Close Impulse and reopen it. You should have the software you purchased in there.

If not, you can always find your numbers on this page: http://forums.wincustomize.com/account/regs 

If it is just the DesktopX runtime cilent you will not be able to update it through Impulse as it is not setup to be used with Impulse. Impulse is incorrectly identifying an update for it.