Maybe you are not serving multimedia for that share in particular? In the dashboard UI, go to ‘Shares’ and check if the symbols showing a movie, music and pictures are in color, that means you will be serving media to other devices.
Otherwise your files will be accessible through FTP/WebDAV/File access, but not in Twonky.
Windows Firewall shouldn’t have anything to do it with, since the media is shared between the MBL and the Xbox.
And can you connect through the web interface to Twonky? You need to input in the browser: http://:9000/ . If you can’t access, i think it means twonky is not running correctly
There you can see if Twonky recognizes the media in the drive, it tells you how many videos, pictures and music files you have, plus many options.