menu support in theme

Having problems adding menu support to the theme.

“The current theme does not natively support menus, but you can use the “Custom Menu” widget to add any menus you create here to the theme’s sidebar.”

I’ve tried adding

function register_my_menus() {
  register_nav_menus(
    array(
      'header-menu' => __( 'Header Menu' ),
      'extra-menu' => __( 'Extra Menu' )
    )
  );
}
add_action( 'init', 'register_my_menus' );

to the functions.php but it hasnt worked.