Well, I don't know what you could be doing worng. I just tested it. I have a phpbb forum located in a folder called "forum"
I created an FTP user called phpbb, gave it a password, and put "forum" in the third box.
I opened my FTP client and entered the following:
site: ftp.mydomain.ext
user: phpbb@mydomain.ext
password: password I assigned
Logged me directly into the "forum" folder.