Clear board cache (delete everything in the /cache/ folder [NOT /phpbb_seo/cache/] except for .htaccess and index.htm
Activate URL rewriting in phpBB ACP.
Activating the Zero duplicate is wise at this point.
Topic URL synchronization: http://www.example.com/phpbb_seo/sync_url.php
I install and configure phpBB-SEO (and other mods) for a fair price and very fast. If you are interested simply send me a private message.