You have to show the Facebook button for the user to click, in order for them to be able to grant the appropriate permissions to your app. If you have our 'Automatic Registration' option enabled, once the user grants permissions, JFBConnect will do the rest to create or log in the user.
If the social network login buttons are not being displayed on your Hikashop checkout page already, you can override your checkout page template to easily include our login easy-tag {JFBCLogin}. For more options on configuring the login button easy-tag, please see JFBConnect backend > Social > Examples section.
-Melissa