Don't you use the GNOME edition of Trisquel? If yes, then "Brightness and
lock" should be in the "System settings"! You can launch it from a terminal
with 'gnome-control-center screen';
You can update with 'apt-get' but not with the "Update manager"? What about
updating with the "Synaptic package manager" (in the "System settings")?
While you are running it, see whether "gnome-control-center" is installed (if
the command above is not found) and whether you have broken packages (click
the "Custom filters" button in th lower-left corner of the window and then
choose "Broken" in the list above).
