If you follow those same instructions provided by Mike in the Creating Multiple Menus when using the TwentyTen Theme? answer. When you get to the section on editing the template is where it gets confusing with Thesis since the templates are not really the standard wordpress templates. Since you said the thesis hooks are little confusing, and they are to many people, I would recommend you try the OpenHook plugin (http://wordpress.org/extend/plugins/thesis-openhook/) and you can add php calls like below anywhere you like. Openhook creates cut and paste boxes for every Thesis hook and you can drop html, php, text anything you wish. Also gives you easy way to remove the more common hook elements like footer attribution etc.
<?php
wp_nav_menu(array(
'container_class' => 'menu-footer',
'theme_location' => 'footer'
));
?>