I assume you're trying to install the screen saver by dragging the .saver file to "/Library/Screen Savers". If this isn't correct, then please give it a try.
If it is correct and you're not being allowed to add anything to the folder, then it's probably a permissions problem. You can attempt to correct this problem through using the Terminal (if you're comfortable with Unix and chmod). Apple released a Repair Privileges utility the other day that resets the permissions on your system to the default state when the system is installed.
Another option is to install the screen saver by dragging the .saver file to the "Library/Screen Savers" folder in your home directory ("~/Library/Screen Savers/"). Open a Finder window, click the Home icon, find the Library folder in it, and off you go. You should definitely have permission to add items to this folder (it is your home directory, after all). If not, then you have some serious permission problems and should seek out the Repair Privileges utility on Apple's web site.
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http://docs.info.apple.com/article.html?artnum=106900>