Debian 10 multi-monitors in a VirtualBox

[Richly]

Hi all.
I followed the directions for installing spin83 multi-monitors add-on from <https://github.com/spin83/multi-monitors-add-on>. The last line says
> Restart the shell and then enable the extension.
I have set up my VirtualBox (with its installed Guest Additions) to support two monitors, and I've restarted the whole machine, but I can't figure out how to "enable the extension."

What I've tried:
1) In Gnome's tweaks:
a) I've set Workspaces span displays.
b) I've looked at Extensions and can't find anything that I recognize as relevant to this issue
2) I've looked at Settings|Devices|Displays, but this shows only one display, "Unknown Display".

Context:
GNOME 3.30.2
uname -a returns
Linux Debian10 4.19.0-6-amd64 #1 SMP Debian 4.19.67-2+deb10u2 (2019-11-11) x86_64 GNU/Linux
Living inside a VirtualBox Version 6.1.2 r135662 (Qt5.6.2) hosted on a Windows 10 machine.

What have I missed?

Thanks,
Richard