I know this a really old post but I've just come across it while trying to solve the identical issue. There are TWO Display Ports on the Dell U2913WM, #4 is DP INPUT and #8 is DP OUTPUT. You want to plug the PC into the #4 DP IN nearer to the power cable. I realised this after 2 hours of tearing my hair out. Hope it helps someone, if not the OP. [IMAGE]  
Hi everyone, I just got my new U2913WM  yesterday. It works great other then it says no display port cable when on the display port input. The monitor works fine with a DVI cable and my PC, but i get no cable error if I try to use a DP cable with ether the PC or Mac. I would like to get one of them to work with the DP cable as my Mac wont pick up the correct monitor resolution when using HDMI.
U2913WM, DP not being detected?