When you have installed, you need to set it up and enable it.
Go to Extensions --> Plugins, search for HikaShop Widgetkit Plugin and open it.
There are a few fields to setup.
Show Quantityfield is used when you show the add-to-cart-button. In front of the button you can show a Quantity field or not.
Force menu-item will be used when you don't use canonical URLS. The link to the product or category will be forced to use this menu-item.