Hello,
I'm not using the VST, I'm using a sequencer, but the answer is effectively the same, I believe. The knobs are assigned to either a Control Change (CC) number or to a real value in the synth like Delay Send.
In the case of the former you say for example knob 1 is CC#3 (or 9, 14, etc) then in the Mod Matrix you say CC#3 controls say delay feedback (for example). If you're in Sequencer mode (which I think you are if you're using the VST) then voice 1 is MIDI 1, the last voice is MIDI 4. So from your sequencer you define a knob called Delay Feedback, MIDI channel 1, Control Change value = 3. That turns the knob on voice 1. MIDI 2, all other things being equal, turns the knob on voice two. Etc.
If you want to take the second example where the knob is set to a real value like Delay Send, you may get lucky and find the CC value here -
http://elektron-users.com/index.php?...=520&Itemid=30
but you're probably best off doing it with a real CC like 3, 9, 14 etc and the mod matrix.
HTH,
CCQ