Yeah, it's kind of a process. You download the map to your computer, then you have to use a program to download it to your unit. Before basecamp there was another garmin program to transfer files but I think they did away with it and you have to use basecamp now.
To make it more confusing you have a choice to download it to the unit or a card (if your unit takes one). If the unit or card is not big enough you can choose just the section you want, on my vista I am limited.
To make it more confusing if you move the map file you downloaded to a different place on your computer it won't work.
If you are downloading the map file to a card it takes a lot longer to do it through the unit, than if the card is plugged directly in the computer.
Once all that's done you have to go into setup maps and make sure you have the maps selected that you want to display.
It's been awhile since I've done it but I think those are the steps I followed.