Each service defined in the metabase includes a MD_LOG_PLUGINS_AVAILABLE key at the /LM/W3SVC/Info. This key contains a comma delimited list of logging modules available for that particular service. In order for your custom logging module to be available to a particular service, its name must be included in this list. Adding the name can be accomplished by using either the IIS user interface or a script that adds the name to the MD_LOG_PLUGINS_AVAILABLE key. If you use a script to add a custom logging module to a service, you should do so with the LogModuleList automation property.
For more information on using scripts to administer IIS, see Using the IIS Admin Objects.