With an open action diagram, first ensure you have no entities selected, then focus your attention on the toolbar. There you will find a button labeled Simulate, upon pressing it will reveal two options: Discrete Event and Monte Carlo.

From the options revealed to you, select either the button labeled Discrete Event to perform a Discrete Event which codifies the behavior of a complex system as an ordered sequence of well-defined events, or a Monte Carlo simulation which extends the functionality of Discrete Event Simulation and allows for realistic analysis of a system or project’s cost, schedule, and performance.