How to Install Fonts Manually in Mountain Lion
To install any new font manually in OS X Mountain Lion, drag its icon into one of the two Fonts folders that you have access to. Why might you want to install them manually? If you install a font via the double-click-and-use-Font Book method, the font will be installed in your Home/Library/Fonts folder, and available only to you.
If you want other users to be able to access the new font, drag the font’s icon to the Fonts subfolder inside the Library folder, which is at the root level of your hard drive. This Fonts folder has universal access. Or, if you usually want to install fonts for everyone, change the Default Install Location in Font Book Preferences (Font Book→Preferences or Command+,).
The Fonts folder in the Library inside the System folder is reserved for OS X and can’t be modified easily. If you try to remove a font from it — or add one, for that matter — you first have to authenticate yourself as an administrator.
Do yourself a favor, and never remove fonts from /System/Library/Fonts. You can really screw up your Mountain Lion operating system if you remove the wrong font. So don’t go messing with the fonts in folders unless you know what you’re doing. Otherwise, just use Font Book, which prevents you from doing anything bad to fonts.