I am also using Nuvi255W with SG/MY V2.70. Downloaded the JCV file together with the TSM V9 Eng (no pain version). I still cannot get JCV function.
Here is what I have done.
1) Copy & rename the TSM V9 download file to gmapsupp.img
2) CopY & rename TSM V9.JCV file to gmapsupp.JCV

These copies are in my SD-card garmin folder.

Is there anything I miss ??

Edit:

Ok..Manage to find out how to enable JCV after playing around with the Thailand Map and JCV file. All i did was to create a JCV folder inside the Garmin folder on My SD card. Thanks for the great map !!!