Add jQuery Slide effect to WordPress Menu

I'd like to add the jQuery Slide effect to my main menu (when someone hovers over About Us, for example, I want the sub-menu to slide downwards). I've been having a few problems trying to figure this out.

The site is:

So far, this is the code I have trying to initialize the effect:

jQuery(document).ready(function($) {
jQuery( ".nav li ul.sub-menu" ).effect( "slide", "down" );

Any ideas as to what I'm doing wrong?