You'll need to do this from your computer.
Download Cyberduck here: http://cyberduck.ch/
There are other programs like Cyberduck you can use, depending on your OS, but Cyberduck is cross platform so the instructions work for both Windows and Mac.
Record the IP address of your ATV2. You can find that from the Apple menu, or from in XBMC\System Info.
Leave your ATV2 at the Apple menu with XBMC closed.
On your computer, launch Cyberduck.
Click on Open Connection.
Select SFTP(SSH File Transfer Protocol) from the drop-down menu.
Enter the IP of your ATV2 in the server field.
Now you should be connected to your ATV2.
Navigate to /private/var/mobile/Library/Preferences/XBMC/addons/plugin.video.ustvnow/resources/lib/
Find the addon.py file in that folder.
Drag it to the desktop. (I suggest saving a backup copy of the addon.py file at this point.)
Open the addon.py file on the desktop with a text editor.
Find line 39, and make the appropriate changes. Don't change anything else.
Drag it from the desktop back to the folder on the ATV2.
Overwrite when it asks you to.
Now launch XBMC and test it.
Hope that helps.