Garmin Mapsource is a management tool - to manage maps, waypoints, tracks and routes on your PC - and for transferring same too and from your GPS. It essentially is the same as the "world base map" that comes with the GPS, which is essentially worthless for anything. Its just a base map - continents, countries, and major cities of the world - almost no road detail.
You do have to acquire additional maps, and using Mapsource, load them from a PC onto the GPS.
South America is one of those places in the world where there is no one good source for detailed maps. For Argentina, you can get excellent auto-routing maps for free from Proyecto Mapear
- as good as they get. However, for other countries, like Ecuador or Bolivia, there's not a really good source for current GPS maps. So you may wind up using Smelly Biker's Wanderlust Worldmaps - Smellybiker's Wanderlust Worldmap • Index page
but you will find lots of missing data and/or hopelessly out-of-date maps - maps showing roads that used to exist, or not showing newer roads that have been in use for the past dozen years.
Its the 3rd world after all - so you just go with the flow. GPS maps and paper maps aren't mutually exclusive - use both if you want to try to get more accuracy - but with both, you'll find that in some countries you'll be riding on both paved and dirt roads missing from both. They just don't have the resources to keep cartography up to date and distributed to the map printers and GPS map makers.
Chiang Mai, Thailand