Category Drop Down Search Form

Trying to add a drop down search that would list all categories. I found get_the_category_list() function but it’s not working. I have the search form on the main site’s home page, WordPress is in a sub-directory. Right now the search form that is working is:

<?php get_search_form(); ?>

which inserts the form from the general-template.php file. I want to make this form the drop down.

I am using Twenty Ten template.

thank you – Carla