I have been playing with my Garmin 276c.
I have Canada Topo V2 and North America city select V 5 software on my computer along with Mapsource

I download the files to a directory and try to open them in Mapsource but the message says these are not valid mapsource files.
What's wrong?

If you have the new version of Mapsource you need to IMPORT an old MPS file as opposed to opening it. The new version uses a new file format but can import and convert the old MPS.

I managed to open some once I converted them to gdb files. The zip files could be opened. The mps even though imported could not be opened.
Wait till winter and I will show you guys a track