Hi there, I'm ribel's old man. I got his message and as he informed you I too have the Medion pocket pc. I must say here though that I use memory map os software for geocaching and I suspect this is where your problem lies. I do not think you can alter the datum in medion's street map it is a basic (albeit good) street finder.
That said, I will play around with the medion software and see if it is possible to alter the datum and come back to you.