Assuming you have the correct multiprocessor licensing:
If you're using Ansys/Workbench 11.0, in simulation, click tools/solve process settings/advanced. You can set the number of processors available to the program and memory settings, which seem to have a huge impact on performance.
Not every ANSYS process will take advantage of multiple processors. When running large problems on my DELL 690, meshing will only grab one. Solver uses all 8.
Doug