CCC-> My Digital Flat-Panels -> Display Color (Digital Flat-Panel)
Then adjust Brightness. This is obviously digital brightness control and not the LCD back light. For real brightness, change it on your LCD's OSD (or button if you don't have an OSD).
Are you trying to match brightness between monitors? If so about the only option you really have is something like the Eye1Display2 colorimeter. Something cheaper like say the Huey pro will help you match color gamma between monitors but not luminosity.