To list available controls, type:

# ddccontrol -p

This probes for available monitors, then takes the first probed monitor and lists controls supported by it. This should give you an output like this (this example uses a Samsung Syncmaster 173P and a nVidia GeForce FX 5700):

ddccontrol version 0.1
Copyright 2004 Oleg I. Vdovikin (oleg-at-cs.msu.su)
Copyright 2004 Nicolas Boichat (nicolas-at-boichat.ch)
This program comes with ABSOLUTELY NO WARRANTY.
You may redistribute copies of this program under the terms of the GNU General Public License.

Device : pci:01:00.0-0
Error while reading write message answer: Bad message
Reading EDID 0x50 failed.
Device : pci:01:00.0-1
Device : pci:01:00.0-2
Detected monitors :
 - Device : pci:01:00.0-1
   DDC/CI supported : Yes
   Monitor Name : Samsung SyncMaster 173P (DVI)
   Input type : Digital
  (Automatically selected)
 - Device : pci:01:00.0-2
   DDC/CI supported : Yes
   Monitor Name : NEC Multisync 2080UX+
   Input type : Analog
Reading EDID and initializing DDC/CI at bus pci:01:00.0-1...
Device : pci:01:00.0-1

EDID readings:
   Plug and Play ID: SAM00E2 [Samsung SyncMaster 173P (DVI)]
   Input type: Digital

= Samsung SyncMaster 173P (DVI)
> Color settings
   > Brightness and Contrast
      > id=brightness, name=Brightness, address=0x10, delay=-1ms, type=0
        supported, value=55, maximum=100
      > id=contrast, name=Contrast, address=0x12, delay=-1ms, type=0
        supported, value=60, maximum=100
      > id=magicbright, name=Magic Bright Mode, address=0xdc, delay=-1ms, type=2
        Possible values:
         > id=text - name=Text, value=1
         > id=internet - name=Internet, value=2
         > id=entertain - name=Entertain, value=3
         > id=custom - name=Custom, value=4
        supported, value=4, maximum=4
> Others
   > Restore defaults
      > id=defaults, name=Restore Factory Defaults, address=0x4, delay=-1ms, type=1
        Possible values:
         > id=default - name=Restore Factory Defaults, value=1
        supported, value=0, maximum=1
      > id=defaultluma, name=Restore Brightness and Contrast, address=0x5, delay=-1ms, type=1
        Possible values:
         > id=default - name=Restore Brightness and Contrast, value=1
        supported, value=0, maximum=1
      > id=settings, name=Settings, address=0xb0, delay=-1ms, type=1
        Possible values:
         > id=store - name=Save Current Settings, value=1
         > id=restorefactory - name=Restore Factory Settings, value=2
        supported, value=0, maximum=2
   > Input settings
      > id=inputsource, name=Input Source Select, address=0x60, delay=-1ms, type=2
        Possible values:
         > id=analog - name=Analog, value=1
         > id=digital - name=Digital, value=3
        supported, value=3, maximum=3
   > Power control
      > id=dpms, name=DPMS Control, address=0xd6, delay=-1ms, type=2
        Possible values:
         > id=on - name=On, value=1
         > id=standby - name=Standby, value=4
        supported, value=1, maximum=4

The lines concerning brightness in the available controls list are:

      > id=brightness, name=Brightness, address=0x10, delay=-1ms, type=0
        supported, value=55, maximum=100

The important elements are address (here: 0x10), current value (55) and maximum value (100).

To change the brightness value to 70/100, type:

ddccontrol -p -r 0x10 -w 70

The last lines of the output should be:

Writing 0x10, 0x46(70)...
Control 0x10: +/70/100 [Brightness]