You need the path to the font. see http://www.wowwiki.com/API_FontInstance_SetFont for example and check clearfont2 for getting a new font inplace or sharedmedia
Thank you. I finally got it working. :) I did try with paths before but your reply made me try even more.
The solution for the path was: "Fonts\\ARIALN.TTF" (I hadn't tried the double slashes before) if anyone else needs it.