I don't know this model, but I assume it came bundled with some Garmin map? Make sure to remove this before trying to upload any other maps or you may lose it, especially if you don't have any disk for it.
Just to clarify: Mapsource can upload and download tracks, waypoints and routes, but you can only send
maps to the GPS (overwriting whatever map is already in there). It only looks like
you can download maps from GPS to MS. What actually happens is that the map tiles installed in the GPS will be selected in MS when you choose to "download" maps. If the same mapset is not installed in MS then this will fail.
Otherwise: have fun!