I explained the method to install Xilinx in a previous post, but that method only works for 32 bit systems. If you followed that procedure in a 64 bit machine it will install you’d not be able to compile projects.
Installing Xilinx for 64 bit linux is quite like the 32 bit method, only that you have to set your working directory to bin/lin64 in your Xilinx ISE directory. There’s a similar setup in this directory for 64 bit machines. So the steps to follow would be,
Notice that you have to run the settings64.sh file instead of the settings32.sh