IE Fix MailChimp Evil Pop-up Form Modal Window

I've implemented the MailChimp evil pop-up form onto this website [login to view URL] which kicks in after 10s. A pop-up modal window displays giving the visitor an opportunity to signup to their newsletter. It works great on Firefox, Chrome, Opera, Safari but unfortunately not for IE8 or IE9. On Internet Explorer it displays the form inline at the top of the site and not in the pop-up modal window. I know it's not an issue with IE in general as this site also utilises the MailChimp evil pop-up form which works perfectly in IE8/9 [login to view URL]