On Samsung N150, the brightness keys currently don't work, neither does the default brightness applet. The brightness can however be regulated through the “setpci” command. This is what this applet does, with a decent interface.
The “setpci” command must be given with superuser rights. This can be done in several ways: for instance by using the command “visudo” to add the line
%admin ALL=NOPASSWD: setpci
in the end of the sudoers file. Notice this may very well be a security treat, depending on the situation.
To load the applet automatically at startup, add a “brightung” entry in the list of applications automatically started at the begin of each session.