After I create a user core with the Create IP Wizard, the associated drivers are not available.
Closing XPS and re-opening it allows the driver repository to be updated; this makes the driver available for use.
This problem has been fixed in the latest EDK 8.1i Service Pack, available at:
http://www.xilinx.com/xlnx/xil_sw_updates_home.jsp
The first service pack containing the fix is EDK 8.1i Service Pack 1.