The setting for DIR_WS_PHPBB is used like this:

Zen Cart attempts to locate a file called "config.php" in the path set for DIR_WS_PHPBB.

So, if you have your forum in the folder "/phpBB", and "/phpBB" is a subdirectory of "/home/users/billy123/public_html", then you need to put this into DIR_WS_PHPBB:

Code:
define('DIR_WS_PHPBB','/home/users/billy123/public_html/forum');
In "some" server environments, simply specifying '/forum' is enough, but not always.