Go to
Service Descriptors > CIMServices.wsdl > CimActivities in the Project Explorer.
From
CimActivities, select the Activity you want to add. Drag and drop this activity into your flow diagram (click at the point where you want to place the activity) in the right pane.
Select
Create Service Task To Invoke Operation from the drop down that is displayed.
Your activity will then get inserted into your flow diagram.