Ok, I took a look at the css in the epub you posted. I don't use Calibre to produce ePubs (apart from a few very rough conversions), so I can't help you with that, but I can point out why you aren't seeing the fonts.
Firstly, the ePub has only got Liberation Serif embedded in it, so you aren't going to see the sans font at all, this may be an issue with how you've configured this plugin.
Secondly, the css code for the fonts is mangled. I don't know if this is an issue with the plugin code, but that seems a possibility.
Here are the edits I needed to make to get Liberation Serif to show up in the viewer:
Original:
Code:
@font-face {font-style: normal;font-family: 'Liberation', serif, sans-serif;font-weight: normal;src: url(LiberationSerif-Regular.ttf);}
Modified:
Code:
@font-face {font-style: normal;font-family: "Liberation Serif";font-weight: normal;src: url(LiberationSerif-Regular.ttf);}
1) The font is referred to as "Liberation Serif" in the class definition, but is called 'Liberation' in the @font-face definition. The font-face defn. needed to be changed to Liberation Serif.
2) The font name is surrounded by single quotes in the font-face defn. These needed to be changed to double quotes.
3) I'm at a loss as to what the ancillary names are doing in the font-face definition ('Liberation',
serif, sans-serif). These needed to be deleted.
All three changes had to be made for the font to show up correctly.
I'd suspect this is an issue with the plugin code needing to be tweaked a bit.