Through the admin panel the default db backup is called phpbb_db_backup.sql or phpbb_db_backup.sql.gz if you chose the gzip compression.
I dont understand why it wont just point to the database in the config file I have just added all the details to it.
It has to point at a database which exists and which has exactly the right username etc.
That database also has to include your users etc if you want your forum to show up. You need to find out from the hosts if they've just made a fresh database (which won't work because phpBB has to populate it in an installation) or if they actually restored your existing database.