I can't answer this question directly but while you wait for someone else I have a few ideas...
From my reading of this page it seems like that is something that is generally set up on install (although I only skimmed it, probably pay to read it properly to be sure). I suspect there is a way of doing it post install though. Maybe have a look on the phpBB support pages or forums or even post there. In my very brief travels over there I did notice something about setting it up in the Administration page?!
As the Appliance page does not mention LDAP at all perhaps LDAP is not something available by default? Being open source I am sure there is a way to get it working, although it may take a little research.
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