You need to register your name and email address on their website. You then receive an email with the voucher, which you can use within 7 days of your birthday (either 7 days before or after). You just need to present the voucher and a form of ID (eg. driver's license).
You need to register before 31 Oct 2011.
The Coffee Cruise is normally about $50 and goes for 2 hours, all the way from Circular Quay up to Manly and back. There's a commentator, free tea/coffee, biscuits, and a box of mini muffin, cake and Tim Tam for each passenger.
I went last Saturday with a friend (our birthdays are within 14 days of each other!) and it was good fun! The commentator was interesting and the weather was great.
Nice, i registered for my wife and i as our birthdays are 7 days apart.