Hi,
Well, there are several ways to go about it. You could configure the Hikashop cart notification" plugin via the Joomla plugins manager to either redirect the user to cart, or display a popup with the "proceed to checkout" button, or even make the notification box clickable to access the checkout. You could also configure the HikaShop cart module via the Joomla modules manager and activate its "proceed to checkout" button setting. You could also create a menu item of the type "HikaShop checkout" to allow the users to access the checkout from a link in your menu area.