The location of the navigation menu is set by the template you have chosen. If it's not working for you, you may need to consider changing templates. You can do this easily by clicking the Style > Change Template link in the Sitebuilder. Then you can click through our extensive collection in the Responsive, Premium or Free categories.
Please note: If you use any of the Responsive templates, the navigation can either be above or below your banner. If you wish to explore this, please follow the steps outlined in this tutorial in the Change navigation location section.