The home directory will be /home/username
The path to the "web directory" or document root is specified in the Domains -> Manage Domains panel. The DreamHost convention is to name the document root directory after the domain/subdomain and places it in the home directory. eg http://example.com/ would have a document root of /home/username/example.com and then http://subdomain.example.com/ would have a document root of /home/username/subdomain.example.com
You neglected to mention how you installed phpBB. It's important to know. With the "One-Click - Easy mode" you don't have access to the phpBB installation at all. Attempts to access or modify the phpBB installation files is out of the question as a result. Making your question moot until you switch to "One-Click - Advanced mode" or a manual install.
"One-Click - Advanced mode" is different. You are asked to install it to a url-path and thus if your document root is /home/username/example.com and you entered http://example.com/[phpbb] for the url-path it is installed to /home/username/example.com/phpbb
If you have an Advanced mode install or manual install, you'll probably need to disable some module by manually changing the configuration files.
Customer since 2000 openvein.org