Re: application_menu -> can't add items to it
Looks like you are trying to add the module application_menu as opposed to creating a module with the menuj id in it.
Use admin->menus to edit , then go to modules inset {menu id=?} into dat section of 'Create new user module ' with suitable name and title press create button it will appear at top of page ,assign and then choose it as module with whatever permissions you like.