Can’t get metabox value (a url) returned using WPAlchemy

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.