Ubuntu 11.10 (which the OP is using) is using Gnome 3 -- gnome apps
have migrated from gconf to Gsettings (with dconf backend). The dconf
backend is binary, so you won't be able to directly change it from a
text editor. If you have dconf-editor installed, navigate to
org -> gnome -> settings-daemon -> plugins -> power
in the left pane and you can edit the power configuration in the right
pane. Alternatively, you can also use gsettings from the command line.
gsettings list-recursively org.gnome.settings-daemon.plugins.power
will list all the keys and their values under this schema. If you want
to change a value of a key, say percentage-critical :
gsettings set org.gnome.settings-daemon.plugins.power
percentage-critical 10