I checked your website and on that page the javascript function which is called by the link is missing. Thus, instead of adding the product to the cart, it uses the link url.
I think I know why you have this javascript function missing.
In the file administrator/components/com_hikashop/helpers/cart.php near line 130 and line 149, you should have the code below:
$doc = JFactory::getDocument();
If you replace each line by the code
$doc =& JFactory::getDocument();
, I think it might fix the problem. If so, we will also add the modification in the next release which is to come soon.