[Plugin: WooCommerce – excelling eCommerce] edit functions.php for Customising checkout fields

Hi
i want to remove some fields from checkout form but i cant
in here explain:
http://wcdocs.woothemes.com/snippets/tutorial-customising-checkout-fields-using-hooks-and-filters/#section-2

in this page say for remove fields add this code in functions.php

// Hook in
add_filter( 'woocommerce_checkout_fields' , 'custom_override_checkout_fields' );

// Our hooked in function - $fields is passed via the filter!
function custom_override_checkout_fields( $fields ) {
     unset($fields['order']['order_comments']);

     return $fields;
}

i test it but no any change
can anybody help me ?

functions.php code :

<?php

/*-----------------------------------------------------------------------------------*/
/* Start WooThemes Functions - Please refrain from editing this section */
/*-----------------------------------------------------------------------------------*/

// Define the theme-specific key to be sent to PressTrends.
define( 'WOO_PRESSTRENDS_THEMEKEY', '80cpmgmo9pspi70ck31cwaapn47s7f6dh' );

// WooFramework init
require_once ( get_template_directory() . '/functions/admin-init.php' );	

/*-----------------------------------------------------------------------------------*/
/* Load the theme-specific files, with support for overriding via a child theme.
/*-----------------------------------------------------------------------------------*/

$includes = array(
				'includes/theme-options.php', 			// Options panel settings and custom settings
				'includes/theme-functions.php', 		// Custom theme functions
				'includes/theme-actions.php', 			// Theme actions & user defined hooks
				'includes/theme-comments.php', 			// Custom comments/pingback loop
				'includes/theme-js.php', 				// Load JavaScript via wp_enqueue_script
				'includes/sidebar-init.php', 			// Initialize widgetized areas
				'includes/theme-widgets.php',			// Theme widgets
				'includes/theme-install.php',			// Theme installation
				'includes/theme-woocommerce.php'			// WooCommerce specific
				);

// Allow child themes/plugins to add widgets to be loaded.
$includes = apply_filters( 'woo_includes', $includes );

foreach ( $includes as $i ) {
	locate_template( $i, true );
}

/*-----------------------------------------------------------------------------------*/
/* You can add custom functions below */
/*-----------------------------------------------------------------------------------*/

/*-----------------------------------------------------------------------------------*/
/* Don't add any code below here or the sky will fall down */
/*-----------------------------------------------------------------------------------*/
?>

http://wordpress.org/extend/plugins/woocommerce/