to be honest with you - the easiest way to install phpbb is just download the zip from
www.phpbb.com and then unzip it and upload it to your server. Once it is uploaded make sure you have a MySQL database setup for it - then follow
www.yourdomain.com/forum/install.php directions and it will install it for you. After that you will most likely have to download the config file it sends you and upload that to ur webserver and then delete the install/ and conrib/ dir's and your all set to go.
It is very simple, but do not do it through Cpanel as it doesn't seem to setup right - download and install it normally.
Regards,
Bill