You can use the shortcut menu to run a diagram. Right-clicking an open diagram and selecting either Simulation > Run or Simulation > Run with Context (if the context of the diagram is a Behavior) will allow you to run the diagram. Cameo Simulation Toolkit will simulate the element, which is the context of the selected diagram, the same way it simulates a Behavior or a Class depending on the element which is the context of diagram. You can also specify the diagram as a target of a Simulation Configuration (see Simulating a Simulation Configuration for more details).
You can also run the diagram with either the Run or Run with Context button on the diagram toolbar.