There's no manual or FAQ I've found for the program so I'm not certain if this is the way they intended it to be used. It does work, though, and I can't see an alternative method of using USB.
- You need ADB installed along with the proper drivers for your device. This varies with platform and phone, so check Google or XDA.
- Enable USB Debugging on your device -- Settings > Applications > Development > USB debugging
- Plug in, start droid VNC server
- On a command line, run these commands:
adb forward tcp:5801 tcp:5801
adb forward tcp:5901 tcp:5901
- Visit http://localhost:5801 with a browser or point your VNC client to localhost:5901
I do notice that performance isn't any better over USB vs a 802.11g, so I'd only recommend going to the trouble of this if accessing your device via WiFi isn't feasible.