I seem to remember it looks for gnome-clock to discover your location, and gives up if that's not found, although the GTK version does unhelpfully just exit without a message. It can't work without knowing your latitude and longitude to work out sunset/sunrise times.
Sample config is at http://jonls.dk/2010/10/redshift-1-6-released/ - I only need to set lat and lon to get it to run, though I also have temp-night set for my personal preference.