First a small disclaimer. The MapSource version is designed for MapSource and not Basecamp!
Still some people will want to use BaseCamp and that is possible if you follow these steps described here.
The MapSource version comes with a folder called BaseCamp IMG.
In that folder is an IMG file that you can put on a USB flash drive. On the drive create a map called "garmin" and put the IMG file in there. That is all you need for BaseCamp to see the map.
When you open BaseCamp you will see FLASH DRIVE on the left side of the scree and the map below it. Just select the map and zoom into the map to see it in detail.
