How to Change the Screen Saver on Your MacBook
Screen savers are a popular item and a great way to customize your MacBook screen. To choose a screen saver, open System Preferences and click the Desktop & Screen Saver icon; then click the Screen Saver tab to display the settings that you see here.
To add a third-party screen saver module so that everyone can use it on a multiuser system, copy it into the Screen Savers folder within the top-level Library folder. To selfishly keep that saver all to yourself, copy it into your user account’s Screen Savers folder (which is located within the Library folder in your Home folder).
To access your Library folder, hold down the Option key while clicking the Go menu on the Finder menu bar; then click the Library menu item that appears.
Click one of the entries in the Screen Savers column to display a thumbnail of the effect. Selecting the Use Random Screen Saver check box runs through ’em all, naturally. You can also test the appearance of the saver module by clicking the Test button; the screen saver runs until you move the mouse or press a key.
To keep tabs on the current time, click the Show with Clock check box to select it. Lion adds a clock display to any screen saver.
Many screen savers allow you to monkey with their settings. If the Options button is enabled (not grayed out), click it to see how you can change the effects.