I have downloaded Basecamp and the Topo trails map from GSPFile Depot. But I do not have an option in Basecamp for a 3D view. This is a brand new computer, and I'm pretty sure my graphics card has the latest driver. Is there something I am missing or not doing? The map from here is just showing red lines where trails are, but nothing else. Can anyone offer assistance?
What you are seeing sounds right. MyTrails is an overlay that only contains the trails and some POI's. On your GPS you would enable MyTrails AND another map of your choice (such as a topo or City Navigator). The trails are then shown on top of the other map.
But Basecamp will not allow you to do this. It can only display one map at a time. If you read the MyTrails instructions, the author describes a procedure for extracting the trails for use in Basecamp along with another map.
Not sure why 3d view doesn't work, I have never used MyTrails. However, note that only Garmin has the tools needed to put 3d features into a map. User contributed maps like the ones on this site will display in 3d but will be flat, whereas Garmin's topo maps actually have mountains that rise up from the valleys.