There are so many things wrong with this plugin I am not sure how it ever worked. In addition since there is no caching and it has to lookup posts for every user, every time, it will bring an active forum to a crawl.
However if you insist, change these two lines...
$path_to_subdirectory= bb_get_option('uri') . "my-plugins/ranks/";
$get_config = $path_to_subdirectory . "config.php";
to this:
$path_to_subdirectory=bb_get_option('uri').trim(str_replace(array(trim(BBPATH,"/\\"),"\\"),array("","/"),dirname(__FILE__)),' /\\').'/ranks/';
$get_config=rtrim(dirname(__FILE__),' /\\').'/ranks/gdr_config.php';
Then you have to edit gdr_config.php
and put the opening and closing for php inside at the top and bottom:
<?php and ?>
Then then the plugin will load.
How it's supposed to run I have no idea.
In theory I could add this ability to my Post Count Plus fairly easily, however I have no time to do that presently and based on the (lack of) donations, it's not going to happen any time soon.