Did you run the installer itself? Do you see ElectricSheep*.saver file either in /Library/Screen Savers or in ~/Library/Screen Savers folder (where ~ is your home directory)? The 2.7b12 did not have installer yet, but the 2.7b15 does so you need to run it before being able to use the screen saver.
Did you run the installer
Did you run the installer itself? Do you see ElectricSheep*.saver file either in /Library/Screen Savers or in ~/Library/Screen Savers folder (where ~ is your home directory)? The 2.7b12 did not have installer yet, but the 2.7b15 does so you need to run it before being able to use the screen saver.