Do you have the virtual camera turned on...? Like.... from the Controls dock, middle one, between "Start Recording" and "studio Mode"...?
Also, doesn't OBS come with Virtual Camera now? So which one are you downloading as a plugin?
I use the Droid Cam Virtual Output plugin, because it was the only one that showed sources from the Exeldro Downstream Keyer plugin. The native one - and others - didn't register anything shown on DSK, so it was my only option. I haven't looked into whether or not that has been fixed, but I've stuck with it.
If you have a plugin like that, you might have to turn it on in another place as well. That one requires going into your Tools options up top, and making it "Active" - not sure if your plugin, if different, requires a similar thing (That one has options for "Active" and "Autostart when OBS is active")