Hi.
It is not clear to me exactly what you are trying to do and what problem you are encountering. Could you be more specific about how you are trying to use the TReX? Do you have the mode jumper set to be in analog or serial mode? If the TReX is in analog mode, you should have to activate serial override before the motor will respond to serial commands. Are you able to control your motor using just analog or serial control alone? Could you post a wiring diagram showing how you have everything connected and a picture of your setup?
Separately, this vi posted by another community member might be helpful.
-Claire