When you create a route in Mapsource on the PC, you are promted to save it, it then becomes the .gdb file with whatever name you gave it. Next time Mapsource is started, it shows nothing of previous routes created. You have to load it using the File/Open option on the menu, or double click on the .gdb file which then opens Mapsource and loads your selected route.
To get that route into the Sat/Nav you either have to load the card into the Garmin card reader, or plug the Sat/Nav into the PC. You then have several options of how to transfer the data from the PC to the Garmin unit. You can go through the menus or there may be two icons on Mapsource, one to upload and one to download, you are looking for the download option. To download your chosen route from the PC to the Garmin unit. The PC will determine which is plugged in...the card or the Garmin unit.
I have an older unit (SPIII) and I find it more reliable to use the card reader to load the specific maps, but plug the unit into the PC to transfer routes and waypoints Etc.
Hope I've got the right end of the stick regarding your question and hope it helps.