I believe two steps will fix your issue:
1. Double click the activity which you intended to model as sub-process, in the type combobox, select subProcess, OK to save it. (It was "abstract task" by default and you didn't set time parameter on it, that's why you got 0 for this "subProcess")
2. Make sure the subProcess is valid. E.g. one simple valid process contains a start event, a Task, an end event.
I've just ran this case and it works:
StartEvent1->Task1->SubProcess1->EndEvent1
enlarge SubProcess1 and put following elements in it:
StartEvent2->Task2->Task3->EndEvent2
Then Configure BPSim Time parameters(ProcessingTime for example) on Task1 (10 minutes), Task2(2 minutes), Task3(3 Minutes);
The result:
Average Time on Task1: 10 Minutes
Average Time on Task2: 2 Minutes
Average Time on Task3: 3 Minutes
Average Time on SubProcess1: 5 Minutes (it is the summary of Task2 and Task3)
Hope that helps

MMA