Selecting the menu block allows you to select a WordPress menu to display on the page. It will present a single dropdown field.

Although this block can be used as part of the main content and in the sidebar the design of this block is optimised for the sidebar.

Menu

Under the dropdown all menus which have been created in WordPress will be displayed. Simply select the menu you would like to use and save.

See the Adding menus article for details on how to add menus to then add to pages.