The other option is that you have the right riser but do not have the RAID key (may have been removed at some point before selling). As Geoff suggested, check the riser, but here is the documentation on activating the RAID controller:
Another possibility is that you have the correct hardware, but it is not enabled. Boot to BIOS Setup (F2), Integrated Devices, and set Embedded RAID to RAID Enabled. If you can't (it is not an option), then you don't have the required hardware.