Magellan uses .poi files on the unit itself and .mgln files on an SD/MMC disk. If you plug your GPS into your PC your GPS is now an external USB drive with files on it. If you look at the files you'll see a .poi file which contains all the POI's from the factory.
Find "Magellan POI Editor". Install the software to add Magellan custom POI's. The software will convert .CSV (comma separated variables - like Excel) files to .MGLN files.
For custom points (.mgln files), load them onto an SD disk. You can get them with a SD USB reader. You can buy a 1 to 4 GB SD disk, plug it into the USB reader then load the custom points. You can then plug the SD disk into the slot on your GPS.
To prevent data loss, don't forget to lock the SD before putting into the slot (there's a small white slide switch on the SD to lock it).