First, open or create your Cubase/Nuendo project.

Click on Devices>Reason.

You will then get a popup window that will allow you to activate your Reason channels

Activate the Reason channels that you will need, then close this window.

Open Reason and create or open a song.

You should now be able to use Reason alongside Cubase/Nuendo, and the audio from Reason will now come thru on specific channels inside the VST mixer.