Resources for using MEGAN
FORTRAN is a general purpose language mainly developed to perform mathematical operations, (compilers may be found at http://www.fortran.com/compilers.html), while Python (http://python.org/) is a general-purpose, high-level language. ArcMAP is a Geographic Information System (GIS) software platform that is used (in the case of MEGAN processing) for the input data preprocessing steps (specifically to average the various input dataset values within each cell of your model domain), and is not needed if a user is programming-savvy enough to write spatial allocation code that can perform the same task without ArcMAP. You can find out more about ArcMAP, including how to download a 60-day trial version at: http://www.esri.com/software/arcgis/arcgis-for-desktop/index.html. It is possible that the free trial version of ArcMAP does not include some of the processing tools needed to complete MEGAN input data preprocessing, but is at least a starting point for becoming familiar with ArcMAP.
You may also find the following online tutorials useful: