The feature (described here:
http://www.dokuwiki.org/doku.php?id=acl#user_wildcards ) that allows users to have different permissions on "personal" pages using @USER@ expansion in conf/acl.auth.php can only be set manually because the ACL management page does not support this yet.
That makes it expand @USER@ to the administrator currently modifying permissions through the manager, saving the result of the expansion instead of keeping @USER@ as is. That's quite annoying for admins not having access to the configuration files.