Hang on, that is for the setup of XN. After that you need this https://www.xigmanas.com/wiki/doku.php?id=documentation:setup_and_user_guide:virtualization_virtualbox

That's the problem I'm having - this is out of date (half way down it states *noVNC is currently broken and removed from XigmaNAS builds, please ignore below.). using a different VNC viewer I couldn't get it to connect though.

While writing this I finally figured out the issue though so to help anyone else who might get stuck with this:

By default on 'remote display' settings for the VM - 'Net Address' is set to 127.0.0.1. This is greyed out and unable to change while VM powered on. If you power off the VM and overwrite it with your Xigmanas IP then power VM back on then I could now get VNC viewer to connect.