The way I do it is go to the map site, pick the map I want, click on it then scroll down to where it says download PC version, Windows version, Mac version...or whatever and click on that. Then follow the instructions and when it's downloaded. I go to my Mapsource open it and on the upper left hand side is a drop down menu, click on it and it shows the download. Then click on it and the map opens.
There are a lot of tutorials on how to do it all on the site. If you don't have mapsource you can get it free on the site also.