Responsive dropdown menu with smaller screen sizes

Looking at that tutorial, you need to match up the jQuery selector with the menu markup. You can see in this line

navigation: $('#nav ul'),

that the jQuery expects your menu to be in a <ul> inside a parent element with the id nav. Luckily, that’s really easy to do. Just change your wp_nav_menu() arguments to:

wp_nav_menu( array( 'theme_location' => 'main', 'container_id'=>'nav' ) );

wp_nav_menu() can do a lot more than that, so you might want to go through the available arguments just for future knowledge.

Also, it seems worth mentioning that the new official WordPress theme Twenty Twelve uses a responsive menu solution that you might want to take a look at.