The quickest way is ACP>General (first page that opens) >view inactive members ( scroll down to the bottom right ) to activate an account .Here is what I am getting from the phpBB forum on activating an account:
ACP>Manage Users> type in users name> Select "activate account" from quick tools drop-down> submit.
I don't find that quick tools drop-down or "activate account."
Errrm, it's not responsive? There's no excuse for such a flaw nowadays, and mobile visitors don't forgive it...