I'm working with Tableau 10 and currently I can see Layout and it lists the sheets currently being used:
New sheets that I have added, do not show up here. I added a new dashboard (just to see if the new sheets would pop up, and they did. How do I make the layout go away so I can access new sheets?
In your first screenshot you are highlighted on "Tablet" layout. Click "Default".
You're encountering new behavior in 10.0 where you can save different dashboard layouts for different devices, all from the same dashboard in your workbook.