Go to Koz's Calibration Corner at:
http://www.kozco.com/calibrat/gray.html
and download a copy.

Look at the dark bar in the center of the chart. This bar has
broad zebra stripes of 5 percent gray over dead black.
Adjust the brightness (the "sunshine" control) on your monitor
down until the entire bar is black, then up until
you can just see the stripes. Display that bar as large as you can.
Big is good. If you never see the zebra stripes, or they're
way too bright always, then the monitor is
adjusted wrong inside or the system "gamma" may be set wrong on your Mac.