I don't think you can use a memory map in Basecamp.
You have to make a custom map from the memory map I guess. At the moment I don't know the format of a memory map.
Maybe you have to make a print, calibrate the map and then use Google Earth to make a Garmin custom map for use on your Etrex.
Maybe it's possible to use Global Mapper, or OKMap or Mobile Atlas to make directly a custom map from a memory map?.