This works only for the detail page. You have to register the meta data in all other main pages too.
Put this in all main php files (index.php, categories.php, lightbox.php, member.php, postcards.php, register.php. search.php, top.php)
$site_template->register_vars(array(
"meta_keywords" => "some keywords",
"meta_description" => "little description"
));
maybe just before:
$site_template->print_template($site_template->parse_template($main_template));