I have a Garmin nuvi 40 LM and although Garmin says it is not a supported device, I wanted to try it anyway. However, no matter what I did, the map would not show up in my GPS.
I found a solution to my problem of installing Garmin Lakes on an SD card for Garmin nuvi 40 LM and wanted to post it, in case someone else has the same issues.

I downloaded and extracted the (15) .rar archives. I unlocked the file "US_GarminLakes_Full.img", using gimgunlock. (like a boss)
Now came the problem... I copied and pasted the US_GarminLakes_Full.img file to the sd card as follows: "sd\Garmin\Map\", as per instructions on this thread. ~ It did not show up in my Garmin 40.
I tried just about everything and was about to give up, when I tried this:
I deleted all the folders on the SD card. I created a folder on the SD card as "Map" (No Garmin folder). I pasted the .img into that folder and it worked like a charm! Just one folder on the root drive named 'Map'. I put other maps into that folder that would not show up before and now they do!
Evidently, this nuvi 40 doesn't use a "Garmin" folder...?

Anyway, I hope this helps someone else with the same or similar problem.

As a side note, I lost my junction view in the process, so it wasn't a complete win/win situation. But since it was the low-res default JCV, I 'll take the trade!