yeah. that is actually /mnt/us in terms of the main system.
Putty can work on windows to access linux systems.
There are guides to SSH to the kindle 3 on the wiki, many threads
and in the USBnetwork readme even maybe there are some clues to the ip's and authentication methods required/options
Get root access. try again
IIRC you can configure root access via USB (check the usbnetworking docs) or be a player and break that puppies WIFI back.