I have a Garmin GPSMAP 60CSx. This post is for other 60CSx owners who need to load a topo map like I did. Hopefully, I can spare you an entire day of trial-and-error, web scouring, video-watching, manual reading, map-loading and hair-pulling, if you're having the same problems I've had.
I have literally spent a day and a half trying to get a basic topo map loaded onto this thing. I tried to load a Tennessee Topo map from this site, which seemed to load properly, except there were never any contour lines, trails, POI's, or secondary roads displayed on the GPS.
I would advise you to use MAPSOURCE instead of BASECAMP. This difference is basically what did the trick. I did everything else (firmware updates on the 60CSx, updates for Basecamp, map updates, etc...). After I downloaded Mapsource, all of my maps I had downloaded were already populated in the drop-down menus (a nice tidbit). I clicked on the map sections I wanted to load from the main map, selected my device, hit the transfer button and waited with baited breath. VOILA! I don't know why it worked, and I frankly don't give a rats'. I just know it worked.
Good Lordy, what a melodrama this has been. I got it, though...