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.
While Native Access is technically capable of activating third-party libraries like Virtual Drumline, we've found the process still has some kinks to be ironed out. Our recommendation is that you activate your VDL library using Native Instruments Service Center application instead. Download an installer for Service Center from Native Instruments.