your system is outdated. MacBrightness requires system 6.0.4 or later.
a vital resource was missing. Try installing a new copy.
this machine does not have a driver for controlling the screen brightness.
there was not enough memory.
a vital resource could not be allocated - most probably there was not enough memory.
there were no valid, saved preferences available. Use the control panel to choose your settings and restart your computer to solve this problem.
there was already a copy of MacBrightness running. The first one to load will be working fine though. (Remove the duplicate copy to solve this problem)