How to add a font in localhost:/System/Library/Fonts?
I'm not sure but I think localhost:/Library/Fonts is for everyone and localhost:/Users/my_name/Library/Fonts is for one user. What is for localhost:/System/Library/Fonts?..
The /System folder is not really for the normal or even the admin to mess with. You could really screw-up your system if you don't know what you are doing. Personally I look around but I try not to mess with anything directly.