My screensaver wont change from its default setting??

Hard to say what might be the cause of this. For me, the screensaver has never worked correctly in Mountain Lion so I'm hoping someone will get round to fixing it.

As a stopgap, I suggest you install caffeine (it's free) which is a neat little trick that prevents your screen saver from starting up and your computer from sleeping. All you need to do is click on a coffee cup icon in your menubar, to start it or stop it.

Triple-click the line below to select it:

~/Library/Preferences/ByHost

Right-click or control-click the highlighted line and select

Services ▹ Open

from the contextual menu. A folder window should open. Inside the folder is a file with a name of the form

com.apple.screensaver.*.plist

where "*" stands for a long string of letters, numbers, and dashes. Select that file and open the Info dialog.

Does the dialog show "You can read and write" in the Sharing & Permissions section?