No video from capture card or webcam?

Trying to set up Composure for a mixed reality project with the camera feed coming in through an Elgato Game Capture card following this documentation but I’m literally stuck at step 1:

I can select the card as a capture device but nothing shows up, and other programs confirm the feed from the card is being viewed. I thought it might be a compatibility issue but some googling seems to confirm at least a few have got the same card to work somehow. Even after restarts I had no luck getting a feed, so I decided to try doing the same on my home computer (so a different system) but with a USB webcam, same thing here, I can select it in the dropdown, but nothing appears.

Is the documentation outdated or am I doing something wrong?