OK. Here is the definitive answer:-
MapSource does not load .csv (or .xls, xlsx, .ods or any other spreadsheet format files) so you need to convert the .csv file into a format that it can. To do this, use a program called PoiEdit (as an aside, PoiEdit is an example of CamelCAPS!). Download and install PoiEdit from http://www.poiedit.com and load the required .csv file into it. If there is an error loading the .csv file, it will tell you and give you the option of loading using a different format. Choose "Garmin Comma Separated".
The waypoints will be loaded and shown alphabetically as text on the right. You can then save the waypoints as a .gpx file. This .gpx file can then be loaded into MapSource and downloaded to your GPS. The .gpx files are also loadable by other GPS's and into Google Earth as well.
My GPSMAP 695 can have up to 3,000 user waypoints so I combined the waypoint and ALA files into a single EXCEL spreadsheet page and downloaded the lot to my Garmin.
My GPSMAP295 only allows 300 so using the DATA SORT options in EXCEL, you can define areas to cull based on latitude or longitude or you can manually delete those waypoints you don't need until you have a file with the number of waypoints you can manage.
Finally, PoiEdit is SHAREWARE, so please make a donation to the author as a mark of appreciation for his work.
I hope this helps.
Regards
Philip