Active Directory Mapper disabled

Hi,
I'm new to certificate mapping, so please bear with me if this is a
simple fix. I'm trying to enable active directory certificate mapping
as mentioned in Q272175: http://support.microsoft.com/kb/272175/

I am able to go into the Master properties box within IIS, select WWW
Service and click Edit. However, when I go to the directory security
tab, the Enable the Windows directory service mapper checkbox is
disabled and greyed out. Am I missing something here?