When you create your route in BM, ensure that at every turn you plan on taking, you double click the route in order to make a way-point. What this does is when the GPS goes to navigate the route, it typically ask shortest or fastest route. Having these extra waypoints, ensure that the GPS can't alter the route you want to use. once you have the route created, save it a a GPX file, ie. here-to-there.gpx Once you connect your gps to your computer, it should be visible in My Computer and seen like another storage drive. Navigate on the Garmin folder and expand until you find the folder marked GPX. Simple paste a copy of the original gpx file into that folder. You then need to add that route upon starting up the GPS per user manual..
On a side note, I still use the discontinued program of mapsource, I find that it is much better at build routes...