To get NetDrive, go to http://www.netdrive.net/download.html for a free for non-commercial home use version. otherwise they'll try to sell it to you. :)
Instead of using the URL or Shared Location from your shares list on the TonidoPlug in the Site IP or URL field in NetDrive, just enter the base url. E.g. http://<username>.tonidoid.com/ .
This should open your root directory of the TonidoPlug, navigate to the USB drive. You can find it under T:\Drive_Root\media (or whatever drive letter you chose). Then you can make a shortcut directly there if you want.
Now you should no longer get the
Mounting drive T: failed
Login failed. Please check the internet connection.
Also check out http://www.tonido.com/forum/viewtopic.php?f=9&t=1410 if you have trouble with the username.