The Layout Tab appears differently depending upon which view you are in. The example to the left shows the Layout Tab as it appears in View as Web Page.
In order to set the default view (the layout this view opens in when selected), choose from the options to the left and then put a tick in the "Set as default layout for this view" check box.
Next, click on the Apply Layout button. From this time forward (until you change the default) the selected default format will be displayed when View as Web Page is opened.