Doug,
I don't think the "M" key is set to the map by default. You must have set yours up that way. Just pressing "M" does nothing on my FSX. What I do is press and release the "Alt" key, then press "W", then the "M" key. It's just the keyboard shortcut for the menu bar at the top of the screen. I suppose it would be easier to go ahead and map the "M" key to the map, but I've been using my method so long that it is just as easy to press Alt, W, M. But, the point being to open the map however you do, and enter the coordinates to where you want to go.