Originally Posted by Danger
it should be:
font-family: 'Tibetan Machine Uni';
Change font class:
font-family: "Tibetan Machine Uni", serif, sans-serif;
The OP is talking of a fonts directory on the Kobo and not in the ePub. So your URL is still pointing to a non-existing place in the ePub to find the font files that aren't there.
For example, if I create a fonts directory on my 650, I can use a URL to reference these fonts outside of the ePub.
This probably won't work on a Kobo. But you can give it a try.