Thanks for writing in! If you have the pro.zip file you just need to install it as a Theme not a plugin. In your wp-admin, hover over Appearance on the left-hand side menu and then click Themes:
On the Themes page, click Add New:
On the Add New Themes page, click on Upload:
Once you've clicked on Upload, you'll then see the option to browse to upload your new theme, click on browse and select the zip you downloaded. Once you've picked your theme, click on Install Now. Wait for the install to complete and activate the Pro theme.
For more detailed instructions please see https://theme.co/apex/forum/t/setup-theme-installation/54
Please note Pro theme doesn't need the Cornerstone plugin and it has its own Pro Editor, so you can delete the Cornerstone plugin safely.
Hope this helps!