After upgrading to the latest version my navigation hover/dropdown has quit working

We temporarily reverted to before the marketpress plugin was upgraded and everything worked properly. When we reverted back to the upgraded plugin the issue reappeared.

Here is some code we think might be the problem:
Uncaught TypeError: Object #<Object> has no method 'on' ajax-cart.js?ver=2.9.0.2:79
mp_ajax_products_list ajax-cart.js?ver=2.9.0.2:79
(anonymous function) ajax-cart.js?ver=2.9.0.2:129
b.extend.ready jquery-1.4.4.min.js?ver=1.4.4:29
u

He said that this is the part that is messing up and overriding I believe:

$(document).on('click', '#mp_product_nav a', function(){
var m = $(this).attr('href').match(/(paged=|page\/)(\d+)/);
var nw_page = m != null ? m[2] : 1;
get_and_insert_products( $('.mp_product_list_refine').serialize() + '&page=' + nw_page );

// scroll to top of list
var pos = $('.mp_list_filter').offset();
$('body').animate({ scrollTop: pos.top-10 });
mp_cart_listeners();
return false;
});