It's been a long time since I worked on that project, but I believe the driver for the DesignTech is interchangeable with that of the Graphtec CE3000 on which it was based. If you're referring to the sign software that you bought with it, that would have been CiberCut or CiberCut Max. I'm not sure who resells that now, but you might start by contacting Michael Vives, the US President of CiberCat.
PM me if you'd like his phone number or email address.
If that doesn't work for you, you can probably use other sign software, such as LXi or Flexi Sign with the Graphtec CE3000 driver.