Sorry that you're having this issue, first thing first, please make sure that your X and Cornerstone is updated (6.2.5 / 3.2.5 respectively) to make sure that we provide you the right screenshot. If you need to update, you can follow our update guide provided here.
Access the Font Manager from X > Cornerstone
While editing the page in Cornerstone, you can also access the Font Manager in the Settings > Fonts
Now, if you do not see this panels on your end. On your admin dashboard, navigate to X > Settings > General and make sure the Fonts is enabled (green lights)
Hope it helps,