To assign the Pro Flight Cessna Trim wheel to the elevator trim control axis in FSX, do the following:

1. Connect the Pro Flight Cessna Trim wheel and launch FSX.
2. Go to Settings > Controls > Control Axis
3. Select "Cessna Trim Wheel" in the Controller type drop down menu.
4. Scroll down to Elevator Trim Axis and click Change Assignment.
5. Rotate the trim wheel 3-5 times in the same direction until the word "Trim" appears in the assignment box, then click OK. 

NOTE: Depending on how your previous axis assignments were configured, you may need to make additional changes, like making sure that there is not a key assigned or a joystick function assigned to the same axis. Delete any conflicts as needed.