building a custom drop-down menu

Hi,

I have had a look on the forums so hopefully I am not asking an existing question.

I am looking to build a customisable drop-down menu in WP (using css styles I have already created) , where the user is able to add and subtract page headings to the drop downs by creating categories on the WP menu page. So far I have managed to do this for a more straight forward sub-navigation menu and pointing the code to that, as follows:

<nav id="sub-navigation">
<?php wp_nav_menu(array('menu' => 'Sub Nav Menu')); ?>
</nav>

The same code is not picking up the styles for the main-navigation drop-downs and I would like some help as to why this is so. I can publish the full code if need be. Many thanks in advance.