Help with adding descriptions to menus

Hi

Being a non-coder, I’m having difficulty resolving the advice on activating menu descriptions into my theme.

The instructions are here: http://bit.ly/9YehDo

We come to the final bit which says:

“This will add the description with in a span element when we tweak our template code to the following:”

<?php
       $walker = new My_Walker;
	wp_nav_menu(array(
                'theme_location' => 'primary-menu',
		'walker' => $walker
	));
?>

I’m using the arras theme. The header.php contains:

<?php arras_above_top_menu() ?>
	<?php
	if ( function_exists('wp_nav_menu') ) {
		wp_nav_menu( array(
			'sort_column' => 'menu_order',
			'menu_class' => 'sf-menu menu clearfix',
			'theme_location' => 'top-menu',
			'container_id' => 'top-menu-content'
		) );
	}
	?>

I have no clue on how to tweak that arras theme code. Can anyone show me the correct markup?

thanks