Apple - Activating screen saver from AppleScript in Mountain Lion

That is a nice way of starting a random screen saver. If, however, you'd simply like to start the active screen saver, you can use this in Mountain Lion:

tell application "System Events" 
    start current screen saver
end tell

This works in ML:

tell application "System Events"
    set ss to screen saver "Random"
    start ss
end tell

"Random" can be replaced with your choice, "Flurry" or "Shell" and so on.

Source Disclosure: http://hintsforums.macworld.com/showthread.php?t=111478

tell application "ScreenSaverEngine" to run