I had the same problem yesterday with my 890GX Extreme4 when I wanted to install my fourth GPU.
As BkkCoins and pixilated8 said, you have to bridge A1 and B17 pins (on the pci-e x1 slot) to make your adapter visible and choose PCI-Express in your BIOS as your default video card.
Last but not least, I had to initiate the GPU with "aticonfig -f --adapter=all --initial" in Ubuntu. I don't know if you have to do something similar in Windows but that's just in case it still doesn't work for you.
Thank you for the reply, I have tried the bridging trick and setting the default video card to pci-express in the bios. no dice. I've also tried using it with a known working card in the pci-e x16 slot. No dice again. I might have a possible defective cable. Going to try it with a different one once I get it in the mail.