Hi,
The Bill me later service of PayPal is available on each customer PayPal account if they subscribe to it.
It's not something which can be selected on the ecommerce solution (HikaShop). If you're subscribed to it, you will be able to select it on the PayPal payment page once you're logged with your PayPal account after the redirection to PayPal done by the checkout of HikaShop.