Did you read this?
Extension snap-ins extend the
functionality of other snap-ins, but
they are not directly added to a
console like stand-alone snap-ins.
Extension snap-ins can add context
menu items, property pages, toolbar
buttons, taskpad tasks, and items to
the namespace of the extended snap-in
(also called the primary snap-in).
Primary snap-ins can themselves extend
the functionality of other snap-ins.
That is, the same snap-in code base
can create a primary snap-in instance
and an extension snap-in instance.
An extension snap-in is loaded only
when the snap-in it extends is loaded
and the feature it extends is used.
For example, when the user displays a
context menu in a stand-alone snap-in,
MMC builds the context menu, prompts
the stand-alone snap-in to add its
items, and then prompts the extension
snap-in to add its items. After all
snap-ins have added their items, MMC
displays the context menu and then
forwards the menu click to the snap-in
that owns the item.
An extension snap-in can extend only
the node types that a stand-alone
snap-in indicates as being extendable.
The extension snap-in declares itself
as a subordinate to the extendable
node types, and then for each
occurrence of those node types in the
console, the console automatically
adds the related snap-in extensions
below it.
It is important to understand that a
node type can represent a scope item,
a standard list view result item, or a
virtual list
view result item added by the primary
snap-in. Consult the documentation for
the primary snap-in to determine what
the node type represents and the
format of its exported data.