You specify what objects enter a process in the definition of Start Events on the BPMN diagram. You must create the objects external to the event definition -- you cannot create them while in the event definition. You typically do this through the Browser/Explorer: for example, by right-mouse clicking on the Definitions selection in the Browser/Explorer and choosing New, and then selecting Object in the list of definition types, and pressing OK.
Once the Object is created, you may drag it into the Start Event definition.
1 Open the Start Event definition (on a BPMN diagram, an event designated as a Start event has an outgoing flow only).
2 Click the Sim Event tab, and in the Generated Objects property, click on Choices.
3 Select one or more Objects and drag-and-drop them into the Generated Objects property list.