Wordpress: Help with tweaking code

[rvnd]

Hello all. First time here. I would appreciate a little help with some php. I'm a novice at php and was following a tutorial to add menu descriptions to my Wordpress 3.1.1 installation.

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

I've done the simple stuff, but I need more undersanding of php to complete the task. The final bit 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?

Please say if you need more code to view if necessary.

thanks