PHPBB have support forums, fill in a request template, will get your question answered more quickly
A quick overview though, you enter you MySQL database name, which you should have already created, wack this in then, your username and password (what you access your database with, ususally you hosting account deatails too) there is also a field for localhost if i remember rightly (contact your hosting company if you do not know this, usually its in your hosting account area somewhere)
Common errors are you either have entered a database name which you have not yet created, therefor it does not exist and cannot write to it, password is incorect or your localhost is wrong. Everything else phpbb does its self.
Again ask in the phpbb support forums as they are dealing with it all the time,
phpBB • View forum - 2.0.x Support Forums
, Tkey - if you are unsure PM me and ill happily help you out.