Thanks for writing in! When you activate the theme, Cornerstone, our very own page builder gets automatically installed and activated as well. This is an addition to the bundled WP Bakery Page Builder. You can check more about Cornerstone from this thread: https://theme.co/apex/forum/t/cornerstone-content-builder-introduction/118/1
If have installed WP Bakery Page Builder, you do not have to register the plugin to be able to use it. You can easily use it one you have created the page. Find the blue button "Backend Editor" or "FrontEnd Editor" as shown below:
And to know how we have integrated this plugin in the theme, please check out this article: https://theme.co/apex/forum/t/extension-visual-composer/76
Hope this helps.