This is very confusing for newcomers and a terrible experience as an introduction to Meta Box. In fact, there is a page available for Meta Box even in the free version, it is just impossible to navigate to, this is the /wp-admin/plugins.php?page=meta-box page. In the free version it would be a much better experience if MB created an item in the sidebar that took you to this same content, just so new users would not misunderstand. And on this page it should say clearly just what you said above: “In the free version, Meta Box does not have the UI to create custom fields.”
This simples statement is not provided anywhere in the plugin description, or even anywhere in the installation page. In fact, the installation page goes immediately from “install the plugin from wordpress.org” to “let’s look at the custom post UI” which is not available at all without further action and even payment.
Please, fix this. Make it easy for new users to get on board and understand what is happening and how to make Meta Box useful!