You should:
Update the firmware.
Format as Fat32 your sd
Create the "Garmin" folder or "Map" folder (capital letter G or M): it depends on device as Neil wrote above.
Enable in your pc to see files extensions to be sure the map naming is correct.
Unlock the .img with Gimgunlock: Jetmouse shouldnt work coz FID is greater than 2200 in this .img
Name your map with the right name (it depends on device)
Copy it in sd.
If all this doesnt work, change sd or may be your nuvi is faulty