See here how to add a page to the main navigation.

There are two ways to add a page to your main navigation.

1. Navigate to Structure → Menus, choose the Navigation Menu and click on Add link. You can re-order menu items by dragging them to the right position.
2. In the Main administrative menu, navigate to Content (admin/content). Find the page you want to add to the navigation and click Edit in that row. The content editing form appears. Click on Menu settings - Add page to Navigation on the right to expand the feature. Check Provide a menu link for the menu options to appear.

For a more detailed explanation with screenshots go to Add Page to Navigtation.