If this is related to your other question, what you've got is a custom taxonomy, so you need either a taxonomy-{taxonomy}.php template (in your case taxonomy-article_topics.php), or just a more general taxonomy.php template.
(also, go back and accept answers to your other questions here if they've been solved!)
UPDATE
sorry, misunderstood your question-
add this to your functions.php template file to show your custom post types on tag pages:
function wpse28145_add_custom_types( $query ) {
if( is_tag() && empty( $query->query_vars['suppress_filters'] ) ) {
// this gets all post types:
$post_types = get_post_types();
// alternately, you can add just specific post types using this line instead of the above:
// $post_types = array( 'post', 'your_custom_type' );
$query->set( 'post_type', $post_types );
return $query;
}
}
add_filter( 'pre_get_posts', 'wpse28145_add_custom_types' );