I don't have TopoActive maps so aren't sure whether it should appear in Express or not, does the map show there? If so look under map options, note the option to install to computer will on;y be there if you've updated the maps on your device, have you done so?
If none of that helps come back so we can explain what to try next.
Yes, the maps have been updated to the last version available atm. Unfortunately, there is no option to install them to the PC, probably it was there in the older version of Garmin Express. Anyway, never mind, I delete them and keep using the openmaps, no problem at all with that! I also called Garmin about this problem, they said they would have got back to me in a few days, like 3 weeks ago! I just give up on it. Thanks, though!