Native Instruments products (and 3rd party libraries like Virtual Drumline) are activated using either Native Access or Service Center.
Some VDL users have found that Service Center, while older, more reliably activates the Virtual Drumline library. It can be downloaded here.
In most cases, Native Access will automatically be installed when you install the latest version of Kontakt 5 Player. If you need to download Native Access, you may do so directly from Native Instruments here.