Hi,
You'll have to allow which document types (page types called nowadays) can be used. See 'child types' tab of the documentation:
http://devnet.kentico.com/docs/7_0/devguide/index.html?document_type_properties.htm
In your case you'll have to allow the root type as a parent of your 'Page (menu item)' type. You can set the root as a allowed parent, or the Page (menu item) as allowed child from both types...