Sometimes for whatever reason you may need to rotate the screen, unfortunately Apple does not provide an obvious switch. To rotate the screen open System Preferences from the dock, and hold down Option and Command as you click on Displays. This will open the Displays menu with an added Rotation dropdown box with Standard, 90°, 180°, and 270° options.
If the above does not work, close System Preferences completely, and hold down Option and Command as you click on System Preferences and then click on Displays without letting go. I can confirm that it works with 10.6.6.