Have you installed your card into the X16 slot of your motherboard and tried it that way?
Edit:
Shut down your computer, make sure the video card is in the PCIeX16 slot, go into your bios and:
In your BIOS make sure your PRIMARY VIDEO CONTROLLER is set to GFX0 - GFX0= primary video controller on a PCIeX16 slot.
GFX0 = Primary video contrller on PCIeX16 slot
GPP = Primary video contrller on PCIeX1 slot
IGFX = onboard display output.
PCI = Primary video controller on PCI slot
See if that works.