He’s talking about when you make a new sequence… That’s the screen AFTER the screen you’re talking about, or it also comes up whenever you manually create a new sequence. Inside there, where it says “available presets’ at the top, There is a Blackmagic Design pulldown. You need to select one ONLY from that set of options. Obviously you need to pick the one that best fits your footage. After that, you should be able to see something on your external monitor.
If you don’t see those sequence settings, you don’t have your BM installed correctly and you need to reinstall the drivers.