Hey...I'm using MS Flight Sim X...hoping they are the same. If you haven't gotten an answer yet, here is how to activate "Trim" in the Controls (MS Flight Sim X) --> Control Axes setting. As expected, choose "Elevator trim axis", and "Change Assignment".
To get the Trim to show in the event window, you NEED to turn the trim wheel multiple times. For me it took about 5 or 6 rotations before it activated.
Saitek recommends 100% sensitivity setting in the Calibration tab, but I find that to be a bit extreme. Right now I have mine set at around 75% with Null at 0% and it seems to be working okay.
Good luck!