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.

A whole group of us (8) at Le Wagon tonight, all with paid subscriptions to Airtable, find the "add element" button missing when building a Record Review interface for an online exercise. Any suggestions please?

Replying to my own question here, thanks to scramblin_pan on Reddit (https://www.reddit.com/r/Airtable/comments/1d936vf/adding_new_fields_in_a_record_review_interface/)
In fact, when there's no "add element" button, you just need to 'mouseover' your fields and click on the little blue "+" which appears. Hope this helps someone!