Hi
2 suggestions for you. First, is you initiate extcap via console, do you run it as root? And is wireshark running as non-root? If yo, you might need capture privileges as suggested by the wireshark wiki page. Second, yes the directories are not very intuitive. Usually if you run it from a build, it will use run/extcap . If you run it from an install, it will use the /usr/local/lib or /usr/lib/... paths.
Have you tried putting it in run/extcap? Also, please ensure that the script is executable.
If everything fails, you could create a small script in each directory with basically the following content:
#!/bin/sh
pwd > /tmp/path.txt
and read out the path, extcap will execute the script, but do nothing, as it will not find the correct output.
regards,
Roland