I guess you have Garmin USB drivers installed properly
First drag the .rgn in Updater.exe, both files must be in the same directory, Updater program will open in your desktop.
Tick USB device.
At this point plug nuvi USB.
Hold the power button, immediately as Garmin logo appears and at the same time press upper left corner.
Updater will display your nuvi model in its window, this is the moment to click ok and Updater will flash firmware.
More here:
[Only registered and activated users can see links. ]

But before trying to flash .rgn,
!) try to reflash the firmware with Webupdater (be sure battery is charged and Garmin USB drivers installed)
2) try to remove from nuvi every .img .GCD .gpx .gpi .zip .vpm files then restart it and see if it works.