Does BlackMagic list this device as OBS-compatible? They don't even list it as QuickTime compatible, the way they do for their DeckLink and Intensity capture devices. I don't see any mention of OBS at the BlackMagic site's pages on this device.
How are you connecting this device to a machine running OBS?
This device comes with its own software so it acts as a switcher. Not really sure what you're trying to do with it in OBS. You might need an additional capture device to capture this one's HDMI output and get it into OBS.
In fact, in many ways OBS is a free, software implementation of most of what this device does (video switching, lower thirds, audio monitoring, etc).
Did you install the software that came with the device?