Here's what I did to install the "Xilinx Platform Cable USB II" on Ubuntu.
First, install the following two packages:
$ sudo aptitude install fxload libusb-dev
Then, download and compile the USB driver for the cable:
$ git clone git://git.zerfleddert.de/usb-driver $ cd usb-driver $ make $ make lib32 $ ./setup_pcusb /opt/Xilinx/14.5/ISE_DS/ISE
If everything worked, you should see the status LED on the programmer light up after it's plugged in.