Hello. I just now saw your post regarding the 3020. The '3020' is what I noticed. I recently purchased one myself.
I had an older computer with XP OS that I had replaced with a newer one. The older one has a parallel port. That is what I am using for my 3020 CNC controller.
I am pretty sure that you cannot use USB to parallel cable adapter to accomplish the interface with the CNC machine. I know I read that somewhere but cannot remember where exactly, but it does make sense that the 25 pin parallel cable cannot possibly get all the signals from the USB connector. For one thing Universal Serial Bus (USB) and Parallel port are just not the same species.
Ok, that said, you can get a parallel port card for your computer quite cheap (Google "parallel port" and see the results). Just make sure that it is compatible with the type of expansion slots that you have available.
Once you have a parallel cable connected, computer to 3020 controller box, you have to set up the pin assignment, etc. in the Mach3 Mill Config - Ports and Pins.
The documentation that I got with my machine was, well sparse doesn't quite describe adequately, but I did find some documentation on-line and downloaded the files. The one that got me going as far as the computer talking to the CNC machine was at:
http://www.marcmart.com/download/CNC%203020%20usermanual.doc It will show the pin selection to use in Mach3.
I hope this helps and is in time for you not to have scrapped you 3020. I like mine, I wanted a small CNC engraving capability with the ability to do 3D stuff on a small scale. (Still working on getting it all going.
Oh, one other thing. The Mach3 software that came with the machine turned out to be pirated software. I have since purchased a legal license from Mach3. I don't understand, do the Chinese stoop to pirating?
Ok, I've rambled enough for now.
Regards,
Tom