I believe truetype font is just any font that your computer uses. You can use any font you want that has the *.ttf extension, which you can get from your "C:WINDOWSFonts" directory, or you can download them online. With XBMP 2.3 you just rename your chosen font to "common-font.ttf" and put it into the "Media" directory which is in your XBMP location on your xbox. Then you have to change the config.xml file for XBMP so it will read that new font by changing the "<usettfuifont>false</usettfuifont>" setting to "true" which is found at the bottom of the configuration.
Thats what I did for my media player and it worked fine with the new font.
Alex