Dimdim screencaster uses a virtual driver to be more efficient in sharing the desktop.The 'Unable to Activate driver  - forcing 8-bit screenshare' message comes when this driver is unavailable for the screencaster. At this stage, desktop will still be shared but will be of slightly lower quality (i.e. not 32 bit)
 a. Go to the Device Manager and check if you can see a 'Mirage Driver' entry under 'Display Adapters’.
 
 Please refer the following Knowledge Base article to reach Device 
 Manager -

 http://help.dimdim.com/activekb/questions/172/

 You can see the entry -

 b. Start desktop sharing again and verify that that 8-bit sharing error still comes.
 c. If the error still comes, close the meeting and uninstall the driver. You will be prompted to restart the machine. Restarting is very important to finish the un-installation.
 d. Install the mirror driver manually as mentioned below.

 You cannot see the entry
 e. Install the mirror driver manually as mentioned below.

 Manually installing a newer version of the mirror driver -

 Link

http://stage.dimdim.com/Build/DFMirage.zip

 Installing the driver - 

 a. Uninstall any existing Mirage driver from Device Manager ( MyComputer > properties > Device Manager) and restart.
 
b. Check that mirage driver is not listed under Display Adapters in Computer > Properties anymore.
 
c. Unzip the installer and double click on "Install.cmd" and wait for sometime, your screen will flick one or two times.

d. Under Computer > Properties > Device Manager > Display Adapters, make sure that properties of Mirage driver mention that it is properly installed.

e. Clean up all the logs under %APPDATA%\Dimdim

f. Please restart your system.

 Try sharing your desktop.