I have just tried the callback and it works for me. Here is what I did.
1) open new sequence file.
2) double click on model path shown at the bottom of the sequence editor. This opens model file associated with the new sequence file.
3) with the process model file active, select Edit>>Sequence File Callbacks
4) select ProcessModelPostStepFailure in the dialog box provided, click the Add button and then the Edit button.
5) Add a message popup to the newly created ProcessModelPostStepFailure.
6) Return to the new sequence file created in step 1 above.
7) select the None adapter and insert a Numeric Limit Test step type.
😎 change the step limits to be 5 and 10. The step will then fail since the default measurement is 0, outside of the limits.
9) run the sequenc
e by selecting Execute>>Single Pass.
The step fails and the ProcessModelPostStepFailure is called, displaying the message popup.
See if this works for you.