In Summer 2017, Native Instruments introduced a new activation program called Native Access. If you're using Kontakt 5 Player (version 5.6.8 or greater), Native Access is the default activation tool that Native Instruments will instruct you to use.
If you are using an older version of Kontakt or are having difficulty with Native Access, you may still activate your library using NI's older activation utility, Service Center. While this isn't technically supported any longer, it should still work in a pinch. Download an installer for Service Center from Native Instruments.