Check the box next to the page or pages you wish to add to the menu.

Your new menu item will appear at the bottom of the menu list.

Click and drag it to where you want it to appear in the list.

If you drop the new menu item so that it is indented, as shown, it will appear as a submenu item to the item above it. You can always click and drag it slightly to the left to make it a standalone menu item. 

When you are finished editing the menu, click "Save Menu". 

Adding New Pages To A Menu