If you are using ModelSim or ISim, it is best you take a look at the scripts generated by Project Navigator (ISE) when you launch a Timing simulation. For example, when using ModelSim, ISE generate a <testbench_name>.tdo DO file. Modelsim uses this file to learn how to compile the required sources and load the SDF file. It is typically a good method to then learn how to do it yourself via custom scripts.
