When using ldapdiff, you have to start it with the -p option
in order to make it use the provided config file.

Example: ldapdiff -p /usr/local/etc/ldapdiff.conf

If you want to make use of the plugin feature, put your
specific code into ${PREFIX}/share/ldapdiff/plugins/skeleton.c
and compile it. Afterwards you have to adapt your config file
accordingly.