To add a widgeted area below header or subnav (if present) in Genesis child theme, Blissful. Don't forget to go to Appearance -> Widgets and place your desired content via a widget in the "Leaderboard Ad" sidebar.

functions.php

<?php
//* Do NOT include the opening php tag

genesis_register_sidebar( array(
  'id'				=> 'leaderboard-ad',
	'name'			=> __( 'Leaderboard Ad', 'blissful' ),
	'description'	=> __( 'This is below header section.', 'blissful' ),
) );

add_action( 'genesis_after_header', 'sk_add_widget_area_below_header', 9 );
function sk_add_widget_area_below_header() {
		if ( is_active_sidebar( 'leaderboard-ad' ) ) {
			printf( '<div %s>', genesis_attr( 'leaderboard-ad' ) );
				dynamic_sidebar( 'leaderboard-ad' );
			echo '</div>';
		}
}