From the announcement page...
If you have already purchased From Camera to Print you don't need to do a thing. You are already entered in the contest. Everyone who has purchased the download at any time prior to December 31, 2007 is automatically in the running.
Just to sweeten the deal we are going to extend the contest entry offer to include the purchase of any video download or DVD available through our store. This includes our Adobe Lightroom Tutorial, Video Journal #14, #15 or #16, or any DVD video. Each purchase counts as an individual entry. The more products that you purchase, the better your chance of winning. (Bundled multiple products count as multi-entries).
I don't know how to word this any more clearly.
Ps: Retroactivity extends only the the CTP tutorial. All entries based on the purchase of other products begins now. I have now added the word "new" to the last sentence to clarify this though.