You need to install cornerstone as it is needed for the theme to work correctly.
If you login to Wordpress Dashboard, you can go to X > Overview and the plugin will be installed automatically for you.
If that doesn't help, would you mind providing us with login credentials so we can take a closer look? Please provide following information:
Set it as Secure Note
- Link to your site
- WordPress Admin username / password
- FTP credentials
All the best!