Ultimately, this isn't all that difficult.  Your main app will need to
define the interface that all of your plug ins will use.  The main app will
look to either a config file, or perhaps a local directory or registry entry
where mutiple config files can be referenced.  That will provide the app
with the name of the plug-in that you need to call.

I'd suggest setting it up so that the main app opens, looks for a list of
plug-ins (registry perhaps), and then calls each one, passing in an object
that contains a representation of the menu structure.  The plug in will add
items to the object representing changes to the menu structure that each
plug-in wants to see.

You could use delegates, or more simply, you could use a command pattern,
whereby the menu item would produce a "command" event that the main app
would send to the plug-in that registered the menu entry.

Your main app should probably create an abstract base class for any windows
that the plug-ins will need to open.  That way, the plug in apps will
inherit from that base class instead of inheriting directly from
windows.forms.form.  That way, you can have some control over the list of
windows that are open, and your main app will have a mechanism for doing
things like sending a "close" event to all windows when the system shuts
down or the user indicates "Exit" from the File menu.