I am now running Mapsource version 6.16.3 under Windows Vista. That is the latest version available from the Garmin website. I found a download site http://www.gawisp.com/perry/mapsource/
to obtain a copy of Mapsource 6.16.1, as Fred Hiltz suggested. I'm hesitating to install it, not knowing if I should first uninstall version 6.16.3, and would doing so delete my user license, etc.
Anyway, I just reproduced my problem. I tried to open the ACA files of the Northern Tierused with my copy of Mapsource 6.16.3. Both the gpx and mps files produced the pop-up message, "! No error occurred", and the file data does not appear in Mapsource. Previously, I was able to get Mapsource 6.16.2 to open the mps file format, so my upgrade to Mapsource 6.16.3 made things worse. I also verified that Mapsource 6.16.3 does open the gpx files that I obtained from American Discovery Trail.
I should add that I recently acquired Basecamp 3.1.3 software from the Garmin website. It was suggested to me by a Garmin rep as a way to view the Rail Trails NE map that I purchased from Garmin. I found that I could "import" the ACA file for Northern Tier into the Basecamp program. That might be an adequate work-around, if Basecamp can do everything that I need. I'm not that familiar with Basecamp at this point, and I'm curious why Garmin appears to have two competing products.