[Plugin: NextGEN Gallery] smooth gallery, how to use with custom fields?

So, im working on a new Theme and i want to integrate the NextGen Smooth Gallery Plugin. That is found here: http://wordpress.org/extend/plugins/nextgen-smooth-gallery/

My Problem is, i want to use it with a custom field. But that doesnt work for some reason.

My Code:

[code]
<?php $gallery = get_post_meta($post->ID, 'gallery', true); ?>
<?php
$gal = '[gallery='.$gallery.']';
$gal = apply_filters('the_content', $gal );
echo $gal;
?>
[/code]

So, when i use the shortcut [gallery=1] now, it works perfectly with the nextgen plugin. but when i use [smooth=1], the smooth gallery shortcut, nothing happens.

how can i resolve that problem?

thanks all in advance!

marc