Well, I tried blocking the "eye" of the remote with my hand, and pressed the up-flame and down-flame and observed that the red light on the receiver no longer blinked. I also tried the same thing by covering the "window" on the receiver and pressed the buttons on the remote and observed no action taking place. So, I am 99% sure that it is a IR remote. I am wondering if the fact that the remote was made in Germany have anything to do with the MX-500 not being able to learn the Mertik Maxitrol commands.
You said that you don't have to enter a device code on the MX-500 to learn the command, and I understand what you are saying. However, just to make sure, I had set the device code to AUX 155, which I thought made sense.

Can you think of any reason why the MX-500 cannot learn the code, or, is there any other trick that could be used to learn these codes to the MX-500. I tried holding down the up-flame key, also tapping the up-flame key, changing the angle that the two remotes as I tried to teach the MX-500, but no matter what I did, the MX-500 never seemed "to see" the commands of the Mertik remote.