Woo price calculator

I have problem to add calculator to WooCommerce. In fact my version of wordpress is 3 and calculator feature is add since version 2. I read about it , it is called woo price calculator.

How can I add to the list of WooCommerce?
or
If there is any way that help me to count the price of product with amount? ex: if the product cost 10 and customer want 3 pieces of the product the price will change to 30.