Hi,
It seems that the problem is actually not with the cart module.
Apparently, it's your template which decides to display the button or not.
You can see that the button is added in a div for the s5_register:
take.ms/Ddd8S
So maybe the template is made to display that area only when the customer is not logged in or something ?
Do you have the same version of your template on both websites ?
If you test with the default template of Joomla, do you have the same problem ?
If you install the 2.5.0 package on your development website, does it work again like the live website ? (you can downgrade, re-upgrade, that's not a problem)