Hey sorry for the late reply. I had checked out another USB Keyboard example from the Xilinx website before(xapp926 Appnote). Since you mention it I guess I can ask about a quirky problem I had with it.
The ready-for-download files work fine. But I tried using the source code in a separate project (my EDK version is 12.3, which isn't compatible with the files given for the project in EDK v9.1i). Now, when I first power on the FPGA, program the bitstream and execute the elf, it is ok. But if I stop and restart the program, the processor waits in an infinite loop, aiting for an acknowledge from the USB controlle, which never comes). This is totally weird and I have no idea why this is happening. Could you shed some light?