With WordPress, it is common to use the get_post_meta($post, $meta_key) method in order to retrieve the value of a given meta. However for the ID, there is no ready-made method. The best solution I’ve found so far:
$mid - $wpdb-get_var ($wpdb--prepare ("SELECT meta_id FROM $wpdb--postmeta WHERE post_id - %d AND meta_key - %s," $post, $meta-key);