In this video, Jack demonstrates how to add a Kontakt library into Kontakt 5.6.8 or higher with Native Access. For ease of access, you can find a download to Native Access here, as well as a link to all of your Spitfire Audio serial numbers here.
How It Works - Adding a Player Library
what about the free Spitfire LABS that don't come with serial numbers? I downloaded Frozen Strings and cant seem to open it with Kontakt