As far as I can see from watching the video…
In the first half, he’s sending an audio signal from a synth via a bus to the sidechain input of Razor. You can do this in Cubase just fine to something like a Waves plugin (go to the sends section in the virtual instrument you’re using), I don’t own Razor unfortunately so I don’t know if you can send to the sidechain in that, I’d be a bit surprised if you couldn’t, but hey.
In the second half, it looks like he’s using internal parameters to modulate the sound, NI may have called the option “sidechain” but it looks like it’s just internal to the synth to modulate the filter 1 section, there’s no external inputs to the synth I can see.
Towards the end, he’s just assigning a midi learn to another internal parameter and using that to modulate the filter, which you can generally do with NI stuff in Cubase just by right clicking on the control and selecting midi learn.
I’m not sure if any of that is going to help - I don’t have Reaktor, I have Reaktor Player 5, but it doesn’t look like that has a sidechain facility. I guess the section you’ve been reading is this?:
If you can send a midi input to Razor, you should be able to send an audio input, is my point, but it looks like the second half of the video is purely internal modulation anyway? Hope some of that is useful, sorry if not