Hi There,
Thanks for writing in! First of all, make sure that you’re fully updated including X theme & Cornerstone. You can check the latest version numbers from here (https://theme.co/apex/forum/t/troubleshooting-version-compatibility/195).
To install Cornerstone automatically, head over to X -> Overview section and wait for few seconds. Then you will see a notice saying Cornerstone automatically installed. You need to enable PHP cURL extension on your server (please check with your hosting provider, if you’re unsure). Cornerstone plugin folder can be found under your plugins folder (/wp-content/plugins/cornerstone/).
Check your PHP version and make sure that you’re using at least PHP v5.6.x to avoid and incompatibility issues.
Let us know how it goes.
Thanks!