I agree with trotsky, I've done it OK but it took 3 attempts to complete the download (it took about 10 attempts for Mapsource 6.1). This seems to be a common thing with Garmins site; at least it is for me.
The download is in the form of a self-extracting zipped file, which should offer to extract to c:\garmin, you then run the updater program to do the actual updating. If your not getting the unzip part it's the download that’s failed.
2.70 fixes more than Garmins letting on, 2.60 would not work with Memory-Map due to an undocumented "feature" (2.50 was fine). The people at MM tried to fix it without success so I emailed the problem to Garmin - haven't even had an acknowledgement but suddenly 2.70 pops up and it's fixed.