As I explained above. When you installed the discrete graphics card, the onboard graphics are disabled; the graphics card provides the video signal. When the onboard graphics is disabled it means it is not providing a video signal. Plug the graphics card in, plug monitor cable to graphics card, boot up as normal and install drivers.
The question is why are you intent on outputting video through onboard graphics when you're installing a discrete graphics card? You could try forcing the option in BIOS by choosing onboard as the primary display, though it seems to defeat the usual purpose of having a discrete graphics card to me.