You want a dvi-i cable, it'll have 4 pins around the bigger pin.
http://www.amazon.co.uk/Cables-ALL-Supreme-DVI-I-Plated/dp/B007RG5AWS The r9 270x usually comes with 1 dvi-i port, even though the dvi-d port may have the extra pin holes, it's not actually wired. So if it doesn't work on one port, you put it in the wrong one.