Sorry for the confusion. Though we may have featured the Cornerstone Custom Icons plugin, we do not handle the support and maintenance for it as it is not a part of our bundled plugins. You can see the list of extensions in our Knowledge Base.
Please forward this to the plugin author, @michaelbourne to see what could be the problem with your setup.
I just tested the latest version though and it works in my test site. Please try clearing all caches and testing for a plugin conflict. It it does not help, please contact the plugin author.