Hi Mark,
I see the issue in IE, but in FF it seemed to be working OK.
However, you do have some errors on the checkout page....
------------------------------------
Timestamp: 09/01/2013 09:37:29
Error: identifier starts immediately after numeric literal
Source File: http://www.dolphyeducationalgames.com/ShoppingCart.aspx
Line: 905, Column: 30
Source Code:
var google_conversion_value = 9$;
---------------------------------------
Timestamp: 09/01/2013 09:39:46
Error: $("#tabs").tabs is not a function
Source File: http://www.dolphyeducationalgames.com/Portals/0/Skins/Dolphy/js/script.js
Line: 59
-----------------------------------
First thing to do is sort these out, then have a look for other stuff, like cookies being blocked and possible errors in the DNN log like Geoff suggests.
Regards,
Dave.