I have a new computer and using a clean version of Illustrator on Mac Maverick. I have installed my custom fonts via the Font Book and pasted the OpenType Fonts and TrueType Fonts to /Library/Application Support/Adobe/Fonts/ but when I open Illustrator and try to Open .ai Artboard with an exisiting graphic that uses one of the said Fonts I get an instant popup Font Problem declaring "The Document uses fonts or characters which are not available or are in a different format than originally specified" the window tells me that there are two fonts (I have installed) that are "Font not found on the system; missing font has been substituded" And do I still wish to open the document. When I do it does not display the correct font.
Put the fonts into Library/fonts. Either the common fonts folder or the one in your library folder.