The "genace.tcl" file is located at: C:\Xilinx\12.1\ISE_DS\EDK\data\xmd\genace.tcl. You can add the following to the "genace.tcl" file (after the Supported Boards Settings comment) to add support for the ML510 board:
# Production ML510
set ml510(jtag_chain_options) "-configdevice devicenr 1 idcode 0x03300093 irlength 14 partname xc5vfx130t"
set ml510(jtag_fpga_position) 1
set ml510(jtag_devices) "xc5vfx130t"
This is scheduled to be fixed in 12.3.