WordPress.org

1. Upload the 'wordpress-css-drop-down-menu' folder to the /wp-content/plugins/ directory
2. Copy the CSS from theme_css to your theme CSS file
3. Activate the plugin through the 'Plugins' menu in WordPress
4. Place <?php wp_css_dropdownmenu(); ?> in your header.php file (or anywhere you want the drop-down menu to render)
5. You may now begin modifying the CSS to suit.
6. Optionally you can visit the Wordpress Options -> CSS Drop Down Admin menu to exclude certain pages from showing.
7. Now comes with the ability to set before and after HTML to wrap around the menu like so: <?php wp_css_dropdownmenu('<div class="menu"><ul>', '</ul></div>'); ?> Or you can have no wrapping whatsoever: <?php wp_css_dropdownmenu('',''); ?>

Just to be clear, will give you default wrapping which is what I gave in the first example. If you want a custom wrapping just use the first example I gave, but be sure to include the menu class.