If you want the Screen Saver only to be available to the current user, drag it to:
~/Library/Screen Savers/
You may need to create the Screen Savers folder if it doesn't already exist. ~ is your home folder.
If you want the Screen Saver to be available to any user of the machine, drag it using an Administrator account to:
/Library/Screen Savers/
All the files and folders in /System aren't for users to modify, they comprise many major parts of Mac OS X.
- proton