If it doesn't show up in Device Manager, then it's probably not related to a dummy plug. The card should show up in device manager as "working properly", but Windows will consider it "disabled" until it senses a monitor attached.

Try replacing your main video card with this one to see if the card is bad or something else. You could have a bad card, a bad driver (try reinstalling the drivers), or a bad motherboard (or the slot is somehow disabled).