You can’t add them directly to the libraries tab.
Here’s one way of loading them into Kontakt.
Click on Load/Save (floppy disk icon) and select Load and navigate to wherever the nki or nkm files are.
Remember in Kontakt Player they will only run for 15 minutes before you need to reload the library, but Pianobook Kontakt libraries will run without limitation in the full version of Kontakt 6 (and K5 if made with that version).
There are other methods.