First, you need to click "like" on the Zynga fan page. After that, watch for any wall postings from Zynga with the phrase "Fan Appreciation Contest". Once you've seen one of these, you have one hour to email email@example.com with the following information:
Shipping Address / Mail Address
Email Phone Number
10 winners will be chosen at random each time that the contest is open. The contest will be open when the Zynga fan page reaches particular milestones in terms of the number of people who have clicked "like" on the fan page.
We're not sure what's in the "Prize Pack", but the contest rules say that it has no retail value. We're sure that it's something fun and exciting in a Zynga game, but we don't know details yet. We'll be sure to share if we find out!
(Source: Zynga Level Up Giveaway website)