UPDATED MAGELLAN INSTRUCTION HERE: http://fishingstatus.com/places/maps/instructions/magellan
You should download the plugin for your Magellan unit and transfer the file to your unit. You can download it here:
You can also convert the GPX or KML file provided in the download to a Magellan file format using GPSBabel. It is free, and easy to use.
Another option would be to use the GPS Visualizer website to convert either the GPX or KML file to Magellan format.