If I understand your situation, you are powering the 153IR with TRACK POWER with the black "Power Selection" rocker switch flipped to the "TRACK POWER" position.
And, with no flashers connected to the 153IR you confirmed that when you wave your hand or run a train in front of the 153IR that you can hear a "click" from the 153IR showing it operating. You should then hear a 2nd "click" a few seconds (settable Delay) after you move your hand away from the 153IR. Now you know the 153IR is getting power and working (the "click" is an internal relay).
In which case, simply connect the red and black wires from the MTH flashers as shown above. For simplicity the diagram only shows one red and one black from the flasher/sound to the 153IR while there are really 2 reds and 2 blacks. In the absence of the actual manuals in hand, the above diagrams are "best" educated guesses; as you may have read in previous OGR posts, Lionel changed the wiring instructions for the 153IR over the various versions. As I've posted in previous threads, I am not aware of any definitive, guaranteed, sure-fire method to identify the version of 153IR based on markings on the 153IR itself. As you may have read, the marking on the left two connections ("Power Supply" or "A U" in you case) are the best indicators we've established to date; in your case the connections to the MTH signal are the same.
If your track has command-voltage (18V AC), there is anecdotal evidence that this may overheat the MTH flasher. In which case apply lower Accessory voltage (e.g., 14V AC) to the "Auxiliary Power" inputs of the 153IR and flip the rocker switch to the "Aux Power" position.
