In the demo version Main menu is located here:

To add or change a Main Menu, go to the Appearance → Menus

Then you can select a menu to edit or create a new menu:

If the Main menu you need is not in the list, just create a new one:

Then tie the new menu to the Main Menu area:

Then you can add menu items. More details about this you can read here: https://en.support.wordpress.com/menus/