If you’re using Keynote on a Mac with OS X Lion (v10.7) or later, you can view the application window in full-screen view, to help you work without distractions. In full-screen view, the Keynote window enlarges to fill the space of your entire screen, moving into a separate space so that you can easily move between Keynote and your desktop.
Choose View > Enter Full Screen or click the Full Screen button in the top-right corner of the Keynote window (looks like two outward-facing arrows).
Choose View > Exit Full Screen.
Move the pointer to the top of the screen to show the menu bar, and then click the Full Screen button in the top-right corner of the screen.
Press Escape on your keyboard.