I do not see that it allows you to enter a latitude and longitude for a destination. but with the above feature you could just watch the location variations and head in the right direction. just like following an address on a street!
Of course the navigator would do this, not the driver. hehehehehee
According to www.gpsinformation.us the Magellan Roadmate units are NOT suitable for geocaching. They are used for driving directions and navigation but from what I can tell you CANNOT enter coordinates into them.
To be able to do what you want to do you need a hand held GPS unit made for use in the outdoors.
I was gonna say that I have the roadmate series for our vehicles and they don't have that capability. The Handheld garmin that I have does, and is perfect for a lat/long input. they have some pretty inexpensive models, check a sporting goods store.
As others have posted, that unit isn't really suitable for geocaching. I have a Garmin 60 CSX and I've been extremely pleased with it. I don't know much about the Magellans, but I do know that the hand held units work better than the car units. www.geocaching.com is a great resource for information about all aspects of geocaching and there are recommendations for units on there. Your aunt might find that site useful to help her get started if she's really interesting in geocaching.