But I'm still lost... I tried "mapgen -n -i" (and some other combinations) at a command prompt, the computer processed something for a few seconds.... I checked starmap.png... and it was still a blank grid.
-i specifies a configuration script; the only full configuration provided is large300.map (the other .map files are included from it). To adjust the output, large300.map in a text editor. mapgen also needs to be told where your fonts (the ones referenced in the configuration) are. So, for example, on this XP box I get the basic large map with: