The project is currently in a state of transition to Github from svn, which explains the lack of issues and presence. http://www.mlpack.org/ has links to all of the old development tools such as Trac (http://www.mlpack.org/trac/) which will be ported to Github soon.
Development of mlpack has been going on for at least 7 years and my impression is that it's pretty mature. It was originally affiliated with a lab at Georgia Tech, where its current maintainer (a friend of mine) is a PhD student. The dual-tree methods are based on his research.