In your hardware simulator, when manually modifying the input pin values it would feel much smoother if the app interpreted a null value as a 0. When I delete a value, thus leaving a null field, I should end up with a 0 rather than an error.
When exploring the results of more complex chips and trying to determine just where the faults lie, an intuitive interface goes a long way.
I don't think the hardware simulator is "theirs", hence you got no answers!
That bit of software you need it only at the beginning of the course, plus, the tests are done mostly with the test scripts..