If plug-in is not registered, it works in DEMO mode - you can do everything, but you will not be able to import more then 5 items at once. In order to unlock its functionality you have to purchase appropriate license and register this extension.
Registration requires license key. See Knowledgebase articles regarding registration issues. You can reset addon's license (due to hardware change etc) in the same way as the main software license.
QuickBooks POS Integration registration: