Try changing your display_url_info
function to
<?php
add_filter('beernews-child-theme_post', 'display_url_info');
function display_url_info() {
// usually needed
global $basic_metabox;
$basic = $basic_metabox->the_meta();
if($basic['post_link_url']) {
echo $basic['post_link_url'];
} else {
echo 'ERROR : "post_link_url" empty';
}
}
?>
And not to be a jerk, but I would like to point out that WP Alchemy is not a plugin but rather a PHP Class. I just do not want other people to get confused.