The link below will take you to a download of a Tutorial I wrote a few years ago on how to get Mapsource and Basecamp (they don't come with Nuvi's) and how to load maps onto any Garmin GPSr.
When you install mine or Dan's map you do it by clicking on an Install file. This installs the map onto you PC, you view it with one of the programs mentioned above and you use those programs to install the map onto your Nuvi.
The reason you lose my map when you load Dan's is because the "gmapsupp.img" file gets written over. You have to install all maps at once, you have to install Dan's and mine at the same time.
The problem I'm having with that is Dans map isn't installed using basecamp or map source so I can't upload them at the same time. His is a zip file and he gives instructions to extract that file to j:\garmin . I guess i could install his map onto a SD card and hours on the actual garmin. That map for north Cumberland on GPS file depot is extreme accurate and I've used it before. I paid $50 for dans map so I'd like to try to use it.