Do you know what directory you are installing the panel to? Is it /Users/<username>/Library/Application Support/Adobe/XMP/Custom File Info Panels/3.0/panels/
Can you try placing the panel inside of /Users/<username>/Library/Application Support/Adobe/XMP/Custom File Info Panels/4.0/panels/ instead. If your panel does not display when you open the FileInfo dialog on your development machine then you may need to adjust the flash player trust file so it is pointing at the 4.0 directory; /Users/<username>/Library/Preferences/Macromedia/Flash Player/#Security/FlashPlayerTrust
In the MXI file update the paths so it deploys to the 4.0 directory.
I will forward this issue onto the Extension Builder team as it seems supporting FileInfo panels between suite versions is not currently available.
I think indeed that FileInfo panels between suite versions is not available. It does install in the 3.0 folder by default. I tweaked the mxi file so it points to 4.0. Then the panel is available but blank ( none form item built ). But to be honest, I have some dependancies that I didn't copy so it may explain the issue. I will check this out. Now at least I know I didn't do something wrong
Thanks for the time and care,
Just to let you know. Finally it worked in CS6. Once in a while, it looks like the panel installation is bugged. But I removed it and re installed it and now everything is fine.