Well there you go Register Plus Redux is the plugin which is adding your new user verification, and by looking at its code there are no filters or hooks which is said because if you make changes to it they will be lost the next time you update, so i would suggest to contact the plugin developer and ask him to add it.
But a quick fix for you would be to add
after line 2938 at register-plus-redux.php
and you can then call it like this:
and your function will run when a user is activated and $user_id will be passed to it.