The ISE MIG generates two folders: example_design and user_design.
What's the difference?
Example Design includes a synthesizable testbench to generate various traffic data patterns to the memory controller that are fully verified in simulation and hardware. This synthesizable testbench is called the Traffic Generator. The example design can be used to observe the behavior of the MIG design both in simulation and hardware. It is also a powerful aid in identifying design and board related problems. Verifying a proper simulation environment and a working board with the example design should be starting points in your design flow.
The User Design has the test bench but it is not synthesizable so can only be used for simulation.
Which one to use?
it's probably best to start with the user guides to answer your questions. See UG388 and UG416 for Spartan-6 and UG406 for Virtex-6. When you read through the user guides you will find that it depends on your requirement whether you would start with the example design and modify it to fit your needs or start with the user design.
How to use it?
create ddr project