The project uses Subversion for revision control.
You can browse the code using a nice web interface provided by Sourceforge here.
To checkout the latest code, use your favorite Subversion implementation. For example, using the traditional implementation, you can type at the command line:
svn checkout https://nona.svn.sourceforge.net/svnroot/nona/trunk nona
Which will download the very latest source code from the repository.