How to get a value/id from dropdown custom field

Hi everybody.
I know that we can create dropdownlist for choose values.
I am using cutom field template plugin.

+I loaded values up to to that dropdownlist by this code ( I reference from wordpress.org forum):

global $wpdb;
$items = $wpdb->get_results("SELECT ID, post_title
FROM $wpdb->posts
where post_type = 'tour'
and post_status = 'publish'
order by post_title ASC");
$i = 0;
foreach ($items as $item) {
	$values[$i] = $item->post_title;
	$valueLabel[$i] =  $item->ID;
	$i++;
}

+ I can show the value of this custom field by this code:
<?php echo get_post_meta( $post->ID, 'related_tour', true );?>

But I can't get the ID of this item which is selected.
How can I do that?