Sibelius has the ability to automatically load Virtual Drumline instruments into hosted AU or VST plug-ins of Kontakt Player allowing for a seamless way of integrating VDL into Sibelius. This functionality was first introduced in Sibelius 5 as a part of its "Soundworld" technology. Depending on the version of Sibelius you're using, only certain versions of Kontakt Player will be recognized as a "Kontakt" style VST or AU plugin. If Sibelius doesn't specify it as a "Kontakt AU or VST" in its Playback Devices window, it will not be able to auto-load your VDL instruments.
Legacy versions of Kontakt 5 are available as a free download from the Native Instruments website. (NI login required)
Still need Kontakt 4 Player? Even though Kontakt 4 Player has now been discontinued by Native Instruments, registered users of Virtual Drumline may request a free installer here.