MapSource has been replaced by BaseCamp, which is available to download for free. As standard, BaseCamp includes a not very detailed basemap, but it should read any maps loaded on the GPS device that's plugged in.
For loading planned routes on the Nuvis, this depends on the model. Some are rather simplified, so don't allow this. Though you can transfer waypoints (aka favourites) to most of them.
I'm not sure which current models allow loading routes.