In order to debug make sure your "Dispach time" is your output values is set as "1"( Send With periodically Scheduled job" ) before issuing output from you output screen.
Then Save the PO.
Put a break point in IDOC_OUTPUT_ORDERS.
Run program RSNAST00 from SE38 with the output type.
Select the correct PO from the list and execute it should stop inside the code.
Regards,
R