actually with these piggyback type units they do not have what most would think is a 'transmission'.
They are driven by a hydraulic motor system.
The main pump and hydrostat control is mounted to the engine, this controls ALL hydraulics on the machine (even the drive).
If you are having drive problems, no travel or slow travel or what appears to be slipping you need to check pump pressure. Also check the control solenoids that control the flow for the drive motor. Also check the swash plate control levers on the pump/hydrostat control and make sure it is moving freely and not slipping.

you will most likely need a service manual to troubleshoot this effectivly.