Don't plug the cable into the port near all the USB, network etc ports.
There should be one below that, that one will override the onboard graphic card.
For example...
Link
The onboard is on the vertical plate near all the other connections, below is an internal card which is horizontal, in this case the horizontal one is the one to connect to.
Oh, I have mine plugged into the lower one, the one that is the graphics card put in by the old owner.