Need help ASAP with a new sound card!

Hi!

I built a pc 5 months ago, with a Asus Maximus V GENE motherboard in it, and when I got Razer Tiamat 7.1 for christmas, I had to buy a 7.1 sound card. So i bought Asus Xonar DS, because it was cheap and had some good reviews.
When i picked it up at the post office earlier, and tried to install it in my PC, it wouldnt fit.
The PCI-bus was incompatible. I read on a site that the Xonar DS used PCI-X, and i have no idea what that is.
When i bought it about a week ago, it only said: PCI-Connection - PCI.
Well, i bought it, since i have 2x PCIe 3.0 x16/8 and one PCIe x4.

My point is, and what i need help to, is which PCI-bus does Xonar DS use?
Would be very glad if someone could help! Thank you in advance!

Picture of the part that doesnt fit: http://i723.photobucket.com/albums/ww238/Steffen095/P_5...
Asus Maximus V GENE: http://rog.asus.com/motherboard/gene/maximus-v-gene/
Asus Xonar DS: http://www.asus.com/Multimedia/Audio_Cards/Xonar_DS/#sp...