You need to split the large gmapsupp.img into parts readable to Mapsource.
This can be done with Gmaptool, look in the sticky index posts in the forum on where to find it and how to do, they are great for finding info!
After having done this you will be able to see the map in Mapsource, how to set up Mapsource can also be found in the forum.
In Mapsource you can select smaller tiles that can be sent to the gps. I guess that 24MB will not be enough for too many square miles of topo maps but it is worth a try.