Ariel has one main menu location “Main Menu”.
This shows up
- in the center, under the top bar/logo,
- or on the top left above the logo
The location depends on the Header Style chosen under Appearance > Customize > Header Style.
To set this up, go to Appearance > Menus, add a new menu, and assign it to the “Main Menu” location. You can add posts, pages, custom links, categories, etc. to this menu. For more information on how to create menus, please read https://codex.wordpress.org/WordPress_Menu_User_Guide.