The MXUnit source code is available in a Subversion repository at Googlecode or as a git repository at Github.
SVN : http://mxunit.googlecode.com/svn/mxunit/trunk/
Github: http://github.com/virtix/mxunit
For Github users, make sure to look at other forks for specific features.
Patches are always welcome! For SVN, we like to get patches generated from SVN. With Github, simply fork MXUnit, hack away, and send a pull request.