montybella, the handheld GPS you've bought is a very basic receiver and its really only for giving you grid references when your out and about.
Garmin have a bit of softare called mapsource. I dont know what you can transfer the to your GPS but mapsource will do the job.
I think you need to registar you gps on the garmin website [
link]
Mapsource should be available to download from the garmin website