I have created a menu item for UserExtranet, but don't really know how to proceed to display it on the frontend. Any help?
Go to Manage Permissions page in backend and check if user you are logged in frontend have extranet access enabled. To make it easier use the search filter and search for the user you want to check access and Extranet Access cell will show you if user can access extranet ( green icon ) or not ( red icon ). If user can access extranet but you still get access denied message that means there is no folder user is allowed to see. From Manage Permissions page click the user name and you'll be redirected to a new page where you can set folders can be viewed by the user. Check folders you want to make visible for the user also check the tasks he can take against files inside folders ( View, Download, Edit, Add, Delete ), tick Recurse into subfolders checkbox only if you want to apply access and choosen tasks for all folders inside main folder, at any level depth and press Apply Permissions button in the toolbar.