Hi,
the path is correct. The .cfg file has to contain paths similar to
/Users/<username>/Library/Application Support/Adobe/XMP/Custom File Info Panels/3.0
/Users/<username>/Library/Application Support/Adobe/XMP/Custom File Info Panels/4.0
In this example it works for custom panels of CS5-6.
If you still see a blank panel you should try to build the panel with another Flex SDK-version, preferably 3.3 for CS4 and 3.6 for CS5/6.
A panel only works in all Suite Apps from CS4-6 when you build it using the CS4 SDK
-- Stefan