MWSWAT 2012 is a plugin for MapWindow, an Open Source GIS system available from http://www.mapwindow.org which runs under the Windows operating system. The MWSWAT 2012 plugin, which is also open source, is downloaded as a Windows installer.
Install MapWindow first, then run the MWSWAT 2012 installer. MWSWAT 2012 will appear in the Plug-ins menu the next time you start MapWindow.
You will also need to download and install the SWAT 2012 Editor, also available from the SWAT Editor page of the SWAT web site. With the current version 1.2 of MWSWAT 2012 you need version 2012.10.0.14 or later of the SWAT 2012 Editor. If you have the 2009 version already installed, you will first have to rename C:\SWAT\SWATEditor as C:\SWAT\SWATEditor2009. Install the editor by unzipping and running Setup. Use the default installation folder so that MWSWAT 2012 can find it.
You can install MWSWAT 2012 as well as MWSWAT and MWSWAT 2009, and make them all available as MapWindow plugins at the same time. (If you wish to keep MWSWAT 2009 you should also update your MWSWAT 2009 plugin to version 2.1 or later so that it can find the moved SWAT Editor.) If you run MWSWAT 2012 on an existing MWSWAT or MWSWAT2012 project it will, after warning you and giving you the opportunity to cancel, convert the Default scenario to SWAT 2012. You can also run MWSWAT or MWSWAT 2009 on an MWSWAT 2012 project and again, after a warning, it will convert the Default scenario to the older version. To do these conversions you must be sure to write all files during step 3 "SWAT Setup and Run". Note that such conversions to (or from) SWAT 2012 will lose any edits you have done on the SWAT input files.
Two example watersheds have been created, San Juan in Mexico and Linthipe in Malawi. Initial geo-processing for these watersheds is described in the geo-processing document, and the use of MWSWAT to set up and run SWAT on them is described in the MWSWAT tutorial document, both on the documents page. MWSWAT 2012 is almost identical in operation to MWSWAT and MWSWAT 2009. The other resources, maps and weather generator files, that you need for them are in two zip archives:
The source code is available from the MapWindow subversion repository at http://svn.mapwindow.org/svnroot/MapWindow4Plugins/MWSWAT. This is best accessed by a tool like Tortoise.