You need to proceed as follows to set the parallel iterations:
Simply select the Iterate link of your subjob to display the related Basic settings view of the Components tab.
Select the Enable parallel execution check box and set the number of executions to be carried out in parallel.
When executing your Job, the number of parallel iterations will be distributed onto the available processors.
Select the Statistics check box of the Run view to show the real time parallel executions on the design workspace.
This feature is especially useful when you need to use the Iterate connection to pass context variables to a Subjob. In that situation, the variables will be read in parallel in the Subjob and thus the processes handled by the Subjob will be simultaneously run using those variables.