Main problem hear is that the a MenuItem Method has to be static. But if you declare a Transform variable as a static variable then you you can't access it in the editor. Then you can't assign a Prefab to it.
From other questions I think I should us a singleton, but I don't know how to implement that into my code.
Since it is a MenuItem you could use Selection.activeObject to retrieve the currently selected prefab from your project view and instantiate it:
Else if you know exactly what is the prefab you want to instantiate, you could put the prefab in the Resources folder and use it:
answered Nov 25 '11 at 10:16 AM