Swamp Witch is a nice font style designed to give a creepy yet nature like effect to any text string.
To install a font unzip the files you have downloaded, then put the font files (.ttf, .otf) into /Library/Fonts . Swamp Witch font integrates quickly into your system and you can easily access the custom glyphs though any text editing application.
Mac OS X comes with pre installed font styles and Swamp Witch will simply be added to your pre existent library. The process is managed through the Font Book application, where you can easily browse and preview your currently installed fonts.
To learn more about dealing with the Font Book application you can easily access the Apple support page HERE.