I think making the Memberships page more prominent is the key. Perhaps move it between Community Builder and Add-Ons in the menu, or even as a sub-menu item under Community Builder to make it more obvious that membership levels refer to the different CB plans you have. It wasn't obvious to me that the Add-Ons could be used "a la cart" with the free version of CB, and that most of them are also included with a CB Pro subscription. Hope this helps!