Add enclosure

Can anyone explain this :
I use a script to add posts and enclosures. The inserting posts works like a charme. Also adding other custom meta fielsds works however when i try to add an enclosure .. the meta field never shows up in the post.

Is there another way to “attach” an enclosure ?

$mypost_id = wp_insert_post( $my_post );
add_post_meta( $mypost_id, ‘enclosure’, $encstring );
$encstring = $article[‘enclosure_url’] . “\n” . $article[‘enclosure_length’] . “\n” . $article[‘enclosure_type’];