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Use the jquery mega menu plugin…easy to use…you’ll have to replace your menu from your theme though..

Else there is the following function from the codex..

<?php wp_dropdown_categories( $args ); ?>

try to use the ‘include’ parameter from the codex

example:

<?php wp_dropdown_categories( array(‘include’ => array(6, 8, 9, 12)) );

Thank you avinash-woodun, I have installed the plugin and it works perfect!