Pending you've registered,
1. Pop your The Dark Knight Blu-ray Disc into your Internet-connected Blu-ray Player at least 15 minutes before the event and log into BD-Live from the main menu.
2. Fire up your Internet-connected computer and go to wblive.warnerbros.com, sign in, and click on the Post Question button in the event to ask a question.
We have word out to Warner Bros as to exactly how we can expect to receive Nolan's end of the discussion (audio track or text chat?), and we'll be sure to update this post if we hear anything more.
AU: One thing Warner failed to mention, of course, is that you need to have the US version of the film, because there's no BD-Live on the AU version. Sorry folks.