Wp phpbb bridge config.php

To start with I have read up loads and still cannot do this.

I have a wordpress site / self hosted
I have a phpbb3 forum / self hosted
Both are in the root of my server in 2 folders ( root /wordpress and root/PHPBB3)
I have installed wp phpbb bridge plugin and in the settings for the plugin ther problem is this : The file config.php does not exists in the path you have entered ) so after hunting websites and trying diffrent paths I still get no luck. Some ppl say the path starts with c:/ but that’s a local file on my pc and I don’t see it been that the path for config.php in FileZilla is /root/PHPBB3/config.php but when entered it comes back wrong can any one help me

I don’t no much CSS or HTML

Any help would be great