RED186
From their email this morning:
Use Promo Code: RED186 on Tuesday October 11th, 2011, at any RedRoomDVD kiosk, to claim your free DVD. The code doesn't work at our Newtown, Waterloo or Neutral Bay stores so check out our one hundred other locations on our website, which is wicked-cool.
You have to return the DVD before 7pm the next day otherwise they'll charge you $2.99/day after that.