Hi,
If you have not added FontAwesome to your theme then it will not work as you need to add the font file first!
So download the font from HERE
Next read how to add the font in this Moodle tutorial: How to add custom fonts in a theme.
As well as adding the font files for FontAwesome you will also need to add the CSS for those fonts too.
The CSS in this case comes with the downloaded files.
Cheers
Mary