Thank you for writing in, can you confirm that your PRO version is 2.3.1 or later? Because that is when we added the Custom Font feature.
Pro 2.3.1, X 6.3.1, Cornerstone 3.3.1 – Notes
If you're not on that version, please update your theme to the latest version.
Updating Your Themes and Plugins
Font Manager can be accessed under PRO > Theme Options
Hope it helps,