I downloaded "My Trails-High Quality Trail and POI Maps Map"...ran the installer on a PC...no problems so far....open MapSource....select "My Trails" from the drop down box in the top left but what is displayed isn't a map....just a grid and some dots (see attached screen capture). Do you know why that is displaying?
You are viewing that from the 700 mile zoom level! Zoom way in on those dots, to perhaps .5 miles. I think you will see they are POI's and trails. I don't use this map, however I can tell you it's transparent, and only contains lines and dots. Many areas will be completely blank. You must use it along with another map, like a topo or City Navigator for it to be useful.
And you can only do that on the GPS itsef because Mapsource can't display more than one map at a time. But your GPS can display multiple maps, so it will make sense there.
Bottom line is that nothing is wrong, that's just the way MyTrails works.