Hi,
You have a javascript error on your pages:
Uncaught TypeError: Cannot set property 'innerHTML' of null on line 21
in the file
www.miobio.at/templates/gk_yourshop/js/gk.scripts.js
You can see it in your browser's javascript error console.
Because of that the javascript of joomla in order to display the popup cannot be triggered and the popup notice doesn't display.
As far as I can see, it's because that javascript code is expecting a cart module of another shopping cart to be there. And since it's not, it produces that error.
So a solution could be to remove that javascript file from the index.php file of your template so that it won't cause any problems. But I would recommend to contact your template provider for advice. They know better than us their template and maybe that there is a better/simpler way to solve the problem.