Why is this not showing?

A feature of Plone is the ability to create a new content type (page, image etc) and hide it from general viewing until you have the item displayed/formated etc just the way you want it. For instance a 'Page' could go through several 'tweaks' before you are happy to make it visible to the general public.  To enable this, when a new content type is created via the "add new" drop down, it is automatically saved in a 'private' state.  Therefore only 'logged in' users (with the appropriate authority) can see it.

The above navigation is a logged in view and at the end is an item called 'A page'. This item has a 'State: Private' on it.  Below is the same website but from a logged out view.

Note that you can no longer see the 'A page' at the end of the navigation list. This means 'A page' can be edited and changed without being visible to the general public. Only when the page is laid out as you require and the 'State' changed to either 'Public Draft' or 'Published' will visitors to the site be able to view this document.

This works well while you are getting the page to look just right, but then when it is ready to be viewed it is important to remember that the state for ALL content on the page needs to be made either 'Public draft' or 'Published' if you want non logged in users to see it.  This is especially important if you have loaded images into your document.  Even if you make the document visible, if you have left the image(s) as 'Private' they (the images) will not be visible, only your text will be.

So if you can see something when you are logged in, but not when you are logged out, check the 'State'.  More often than not, you will find that this is still set to 'Private'.

For instance, on the 'Contents' display here, the majority of the items listed are 'Published' (viewable to all). However, the item called 'A page' is set at 'Private'. 

This means that when logged in, the 'A page' will be visible and you can access the page, but if you are logged out the link will not appear in the top navigation. It also means that if you are sent the link to access the page, and click on the link while logged out, you will get this screen:

A request to login is absolute proof that the item has a 'State: Private' on it.  If it is meant for general viewing, change the state to a public viewable option (Public Draft or Published).