1) Open the plugin file and replace the name of the function updatethreadcount with update_thread_count.
2) Try this: Open the plugin file and search for:
$message = str_replace('{username}', htmlspecialchars_uni($username), $mybb->settings['postonreg_message']);
$subject = str_replace('{username}', htmlspecialchars_uni($username), $mybb->settings['postonreg_subject']);
Replace with:
$message = str_replace('{username}', htmlspecialchars_uni($mybb->user['username']), $mybb->settings['postonreg_message']);
$subject = str_replace('{username}', htmlspecialchars_uni($mybb->user['username']), $mybb->settings['postonreg_subject']);