Navigate to Appearance > Menus and create your main menu. Inside the “Manage Locations” tab you can assign this menu as the Header Menu.
The theme features two menu locations: Primary menu.
To create a menu item linking to a section of the One Page template select Links from the left side panel and set the “URL” to #page-slug and “Link text” as the name of the menu item (eg. #about-us as URL and “About us” as Link text).
The left side panel box should look like this: