To add the navigation menu, click on Menus under the Appearance tab in your WordPress admin. Start by creating a new menu in the panel on the right. Once the new menu is created, you can add pages, categories and custom links from the panels on the left.
Adding Pages to the Navigation:
When you publish a static page to your site, you will need to add it to the navigation menu manually. For example, let's say you created a contact page that is now live. You will need to come to the "Menus" page to add it to the navigation. Static pages will appear in the "Pages" box on the bottom left of the page. Check the boxes to the pages you would like to add, and click the "Add to Menu" button. The pages you selected will be transferred to the right side of the page in the navigation box. You can then re-order and create submenus by dragging the menu items. When you're finished, click the “Save Menu” button.
To change the navigation menu location you want to go to Theme Options under Appearance in WordPress. Click the Header tab and select the Navigation Location.
Tutorial: How to Center Your Navigation Menu