I guess you haven't followed with care this:
Before doing anything Drag&Drop the rgn file you want to flash to your device (it can be a normal firmware file or a file rgn extracted with RGN Tool or you got from Garmin ) in Updater.exe and it will open in your desktop.Tick USB device
posted here:
[Only registered and activated users can see links. ]
Updater will display your nuvi model in it's window, this is the moment to press ok
You have few seconds to flash rgn , or nuvi reboots.