You have to use the Kobo Tweaks plugin
to use the frontlight program. After you install the plugin, add these lines to kobotweaks.ini (in the .kobo folder on your Glo):
That assumes that the "frontlight" program is in the main folder on your Glo.
Just a note: while this program does let you set the frontlight to a lower level, there is really hardly any difference at all between the Glo's default lowest brightness and this program's lowest.