Insert it as a Soft Synth with a MIDI Source Track. Enter notes using a Controller, Staff View or PRV in the MIDI track. That's how I did it and it works fine. I didn't try inserting it as an Instrument Track but I don't know why that wouldn't also work. I put the 64 bit .dll file in C:\Program Files\Cakewalk\VstPlugins .
Virgin Choir, like all audio plug-ins, will only work in an Audio Track and you need a MIDI track to send data to the synth on the Audio Track. An Instrument Track is a combined MIDI and Audio track but you can't do as much with the MIDI in an Instrument track as you can with a MIDI track and Audio Track paired together. Try it both ways and see what works best for you.