Post the ad where you bought it from, did it come with any software?
In mine, it has a parallel to usb cable that connects through to PC with USBCNC software which I bought (it came with a pirated version which didn't work), it is a good easy program. I haven't seen Mach 3 connected to USB, I tried it and couldn't get it to work its an old outdated program that looks like ****.