To add a button to the main menu navigation area, you can simply navigate to the menus section in your WordPress Dashboard.
Appearance -> Menus
Make sure you are editing the menu that you have assigned to the Primary Menu area.
At the top of the screen, click to open the Screen Options area so that you can turn on the ability to give the menu items a CSS class, and/or a link target.
Turn on the Link Target and CSS Classes checkboxes.
Then when you add a new menu item, or open an existing item, you’ll see the new option to add any CSS Classes you like.
Enter the class guten-button for that menu item to display as a button in the main menu area.