From World Wind Wiki
The Layer Manager allows you to manage various data and imagery sets. It provides finer control over sublayers and individual items that is not availble from the toolbar.
 Accessing the Layer Manager
The Layer Manager is hidden from view the first time you run World Wind.
You can access the Layer Manager by pressing the shortcut key
L or from the View menubar item at the top of the window or by using the Layer Manager toolbar icon. The menu should then become visible to the left side of your screen.