It sounds like what you're asking for is not for a script to navigate to a particular field, but to set a value to whichever field is active at the time the button is clicked. Have you tried using the Insert Calculated Result  script step? You could write a script that is simply:
Insert Calculated Result [Get ( ScriptParameter )]
... then assign each button to call this script with the digit for that button as the script parameter. To accomodate a backspace button and smooth handling of entering negative numbers, you may want to consider something slightly more complicated using the Set Field By Name script step in combination with the Get ( ActiveFieldName ) function. If you do this, remember to wrap Get ( ActiveFieldName ) with GetFieldName ( GetField ( ... ) ) to get the fully-qualified Table::fieldName needed by the Set Field By Name script step.