Go to the Applications folder and run the FontBook app (located in Applications/Utilities folder) to check whether the font is listed - and to resolve any duplicates. FontBook Help can guide you through the process. If you do find any duplicates you must restart your system after resolving them. If your font isn't there, it probably hasn't been installed correctly. If font-related problems persist, you may have a damaged font file – but FontBook won’t tell you if a font file is damaged. To isolate a damaged font, create a folder, then:
1. Move about half of the fonts there by families (and it's better to restart);
2. If the problem persists, it lies in one of the active fonts; if not, swap the fonts between the active folder and those in the isolation folder;
3. Colour the fonts in the isolation folder with green; and
4. Repeat from step 1
In this way, you'll be testing about 1/2 of the fonts, then 1/4, 1/8, 1/16, 1/32, etc., thus working through all the fonts in a few iterations. It's possible that you will still get an error after this, in which case you either have more than one corrupt font, or the problem lies elsewhere.

__________________
Cheers,
Paul Edstein
[Fmr MS MVP - Word]