In version 0.9 I finally included an advanced meta search box. It can be easily styled and you can even have several boxes on the same page.
The code to display it is simple:
<?php meta_filter_box<!---->() ?>.
If you don’t want to use a certain key as a filter, you can write
<?php meta_filter_box($exclude = array('key1', 'key2')) ?>
Make sure your theme has wp_footer() written somewhere in footer.php.