You must have 3.1i with the latest service pack (or the latest WebPack JTAG Programmer) to perform JTAG operations on XC18V00 devices. When you initialize the JTAG chain (File-> initialize chain) the 1800 device will be identified. Double click on it and the software will prompt you for the MCS or EXO file to associate with that PROM. You may now program or verify or erase the PROM. For more details on Programming Options for the XC18V00, see (Xilinx Answer 8411).