Yes, sorry about that. We have not yet fully completed the user interface implementation for the evaluator dialogs, so the experience right now is pretty terrible.
If you just want to use a simple field (no complex calculations), just enter the field name in the Result box. If the Language is VBScript, put square brackets around the field name: [Time_Min]. If the Language is Python, put exclamation points around the field name: !TIme_Min!