My Grass gives me this error message, Grass_info_error(4816,1):'make' required. Please install 'make' first. This happens when I attempt to fetch an add-on (r.viewshed). What am I doing wrong? and what is 'make'? Thanks for helping for a Grass newbie. I'm running on Mac Snow Leopard if that makes any difference.
You may install GRASS 7 in parallel, it contains r.viewshed (so, no need to install it as addon): http://grass.osgeo.org/download/software/mac-osx/