Go to his YouTube page by following THIS LINK and you'll get all the info. You'll need to upload your finished video there as well.
Here is what it says on the page...
Welcome to the WATCHMEN "Veidt Enterprises Advertising Contest." Director Zack Snyder is offering you the opportunity to create "Veidt Enterprises" TV commercials that could end up in the feature film WATCHMEN, scheduled to be released in theaters on March 6, 2009.
To get started, download the standard definition and/or high definition assets from the links below, and then shoot your own original TV commercial that advertises one of the products! Remember, the movie is set in 1985, so TV commercials should be in 4:3 aspect ratio (so it looks like standard TV, not widescreen) and they should be either 15, 30 or 60 seconds in length. The products are all made by "Veidt Enterprises," and you can download the product logos for your use as well.
Make your TV commercial in NTSC standard definition (720x486) or higher resolution, upload it, and then others can vote on your work. The contest ends on June 2, 2008. The 40 finalists will win signed posters from Zack Snyder. The top five finalists from user votes will win High-Definition Canon Vixia HG10 camcorders. Up to 20 of Zack's favorite finalists will win $1000.00, and have a chance to be included in the WATCHMEN feature film itself.