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@Sylari I could force the page into the menu, but I personally don't like that approach. Some people may not want Magic Preview in their blog's menu, and if it's forced in, there's no user-friendly way to get rid of it. (One has to change the plug-ins code or hide the menu item with CSS.)
You can always add a link to the menu yourself. And, as @manton mentioned, this will hopefully be a lot easier in the future.
If I'm reading you correctly, you just figured out the URL on your own by guessing? Impressive! The URL is mentioned in the plug-in's documentation, but it's easy to miss. Maybe the documentation will be more prominent some day.