To start a simulation, select Run, and then Start [Simulation], 
.
You can then stop, continue, single step, or reset the simulation.
After you start the simulation, the Run, then Start [Simulation] command changes to Stop [Simulation]. If you then select Stop [Simulation], the simulation halts and the command changes to Continue Simulation (in Microsoft Excel 2003 or earlier). Choosing Continue Simulation restarts the simulation where it left off, and changes the menu command back to Stop [Simulation].
(In Microsoft Excel 2007 or later, select Run, and then Start to continue the simulation.)