Are you using a USB hub? Is the USB hub passive(no power) or powered(has a power supply)? I suggest if at all possible plugging the MyDMX hardware directly into the computer. Short of that, since it must be USB powered, a powered USB 2.0 hub would be my advise.
Also, try a different USB cable. There was a report of a batch of bad cables. I found my cable that came with my purchase of MyDMX, but in my case, it has been found to be operating within operational expectations. Even so, I don't use that cable because I have a longer one that is more flexible and durable that I prefer to use. Most people seem to have an A-B USB cable laying around due to their various purchases of computer hardware, so hopefully this is your situation as well. Otherwise, you'll need to purchase a cable or borrow one. I'd suggest borrowing one for the time being if you need to test.
You did do the installs as a local or network administrator, right? You also did the installs with the hardware not plugged in, right? Make sure you totally uninstall MyDMX, reboot. Then re-install using the latest stuff via the web site.
Report back. Hopefully you have some news or insights on your situation.