Mark,
You can access the error that caused the StationPostStepRuntimeError callback to be executed by using the lookup strings "Parameters.Step.Result.Error.Code" and "Parameters.Step.Result.Error.Msg". The Station Callbacks all receive a Step container with the information from the calling step as a parameter. You can use the "Parameters.Step" to access information about the calling step.
Feel free to let us know about any other questions. As Khalid said, it is usually better to post in the TestStand section of the Discussion Forums.
Regards,
Shannon R.
Applications Engineer
National Instruments