Looks like you have a conflicting script.
When i test your page and look in the free bug error console, i see the following error:
TypeError: undefined is not a function (evaluating '$('ul.menu').superfish')
the error occurs in the scripts.js file in the following code:
$(document).ready(function() {
$('ul.menu').superfish({
delay: 600, // one second delay on mouseout
animation: {opacity:'show',height:'show'}, // fade-in and slide-down animation
speed: 'normal', // faster animation speed
autoArrows: false, // disable generation of arrow mark-up
dropShadows: false // disable drop shadows
});
});
try removing the scripts .js file from the checkout page.