You turn on the multi-process capability for ST-Sim using the menu option File-Library Definitions. We suggest setting the number of jobs to be at most the number of cores on your machine (for overnight runs); alternatively you might want to save one core for doing other things while the runs are underway. This will break up your simulations into jobs according to scenarios and iterations. You can determine the number of cores on you machine by opening the Performance tab under the Windows Task Manager and counting the number of panels under "CPU Usage History".
Note that you can further divide your simulations into jobs according to secondary strata using the Advanced-Processing scenario property. In general this latter option would only make sense if you had more cores available than the product of scenarios X iterations, and your secondary strata are independent of each other during the simulations.
Hope this helps – Colin.