The referee box requires GTK+ and gtkmm libraries. On Linux, you can probably install them with your distribution’s package manager (you may need to install separate -dev packages on some distributions). On Windows, you can get GTK+ here and gtkmm here.
Releases from the year 2013 and newer can be found on the GitHub releases page.
Development happens in the GitHub-hosted Git repository. Please submit any bugs or patches there as issues or pull requests respectively.
A few old releases are archived here, from before the Git repository existed: