Right click anywhere in a blank area under the menu bar at the top and
select the advanced toolbar. Alternatly you can access this through the View
Menu - Toolbars - Advanced.
The sixth button from the right is the preview pane button.
The other thing to try is to go into View menu - Toolbars - Customize, click
on the Options tab and click the Reset My Usage Data button. While you are
there put a check in the Always show full menus check box.