This is only a DEMO, normally this page will only be visible to you if you are logged in as admin. You can try clicking the "Edit" icon next to the example page, which will show you what the editor looks like. This DEMO is viewable without admin rights, but normally this will not be possible.
To add link to your custom page in your menu, use the syntax for external link. The URL of your custom page is:
where the XXXXXX is the namespace of that particular page you want to use.