It's past May 17th and I still don't have the left navigation UI referenced in the help article "Getting started with Airtable Interface Designer." I'm missing the "add element" button. Anyone else with this issue?
My guess is that when you set up your first interface page, you started with one of the suggested formats of "List", "Kanban" or other options in the top half of the "Getting Started" menu. Those do not have the option to add additional elements. Restart and choose Record Review, Record Picker, Dashboard, or Blank.