How to connect USB hardware to a MBP via an external monitor
I think your problem is that USB is not carried over Display Port/DVI. I have an older Apple Cinema Display that has USB ports on it and the cable has both DVI and USB ends that get plugged into the Mac. 

That DELL monitor has an upstream USB port (and I would be very surprised if that is not connected to your docking station for the PC). Connect a cable between that and the Mac and the keyboard and mouse should work.