Driver:
It sounds like you have not specified an 'output' directory. I think
you shouldn't have to (just creating a directory named 'output' adjacent
to the mapnik xml should be a reasonable default) but there was a bug in
the command-line argument parsing that prevents this from working. I
have updated the git repo with a fix for this as well.
Thanks for letting me know about these problems, I hope you're able to
use mn2gt after this.
--
David Winslow
OpenGeo - http://opengeo.org/