Sounds simple, but did you update the TIR software to the latest version? Because it doesn't show up in controls, I'm pretty sure the game isn't recognizing it. Do you get a blue light on the TIR when the sim starts (not the main menu)?
Edit: I may have misunderstood, I think what you said is you DO see TIR under controls, but it's not assigned anything. In that case the game sees that you have it. This is normal. I would still try updating.