GRASS 6.4+: install with
  g.extension r.seg

---------------------
Old method:
r.seg GRASS GIS module for image segmentation and edge detection 
	Alfonso Vitti <alfonso.vitti [at] ing.unitn.it>
	see www.ing.unitn.it/~vittia/sw

copy the "r.seg" directory in the "raster" directory of the GRASS source code directory

from within the "r.seg" directory, as normal user run: 
make

from the GRASS main source code directory, as root run:
make install
or, as normal user:
sudo make install