Link to comment on a comment

I am making a private blog for my wife and can't find out how, on single post pages where the comments appear, to give readers the choice between commenting on the blog article or on the comment. I have threaded comments enabled in WP, but there is no apparent way to add a 'reply to this comment' function so that nested comments are actually related on the page.

Sorry, can't put link and login details up here but happy to do so in an email so that we can get this sorted. (The blog is visible only to registered users and public registrations are closed).