To add a page to the menu, create a basic page.
Go to the page you would like to add to the menu, and click the "Edit" tab.
Scroll to the bottom of the page, click on "Menu settings," and select the checkbox for "Provide a menu link."
Fill in "Menu link title" with the text that you would like to show up on the menu, and make sure that the parent item is set as <Main menu>.
Save your setting before navigating to another page.