The right path is the full path to you config.php
In example in my blog is somethink like that:
/home/public_html/phpBB3/config.php
in your case may is somethink like that
/home/public_html/forum/config.php
but I can’t be sure about your server a I don’t know the configuration of your server.
If you are not sure what is the full path to your phpBB3 then create a file “ie: mypath.php” into the folder phpBB3 and enter the following code in it:
echo $_SERVER['SCRIPT_FILENAME'];
The above code will return to you the full path to phpBB3 + the file name “mypath.php”. Replace the “mypath.php” with the “config.php” and then copy all that path to use it in the plugin configuration.
Hope this help you ??