MWAGNPS is a plugin for MapWindow, an Open Source GIS system available from http://www.mapwindow.org which runs under the Windows operating system. The MWAGNPS plugin, which is also open source, is downloaded as a Windows installer.
Install MapWindow first, then run the MWAGNPS installer. MWAGNPS will appear in the Plug-ins menu the next time you start MapWindow.
An example watershed, Duffins Creek in Ontario, Canada, has been created. The use of MWAGNPS to set up and run AGNPS on it is described in the MWAGNPS tutorial document on the documents page. The maps that you need for the example are in a zip archive
The source code is available from the MapWindow subversion repository at http://svn.mapwindow.org/svnroot/MapWindow4Plugins/MWAGNPS. This is best accessed by a tool like Tortoise.
There is also source and technical documentation for AGNPS available as a zip archive.