Instructions:Go to the Coke Zone website and sign up and it should automatically take you to a very short 'survey' called 'Spill the beans and count the points' which rewards you with 5 points for completing. You can find the survey on the bottom right of the screen if you are already a member and havn't already done it. The survey is simply filling in your address and answering a few questions about Coke (which you drink / which you prefer, etc).
The 2 for 1 vouchers on the site are 5 points each. Usually to get the points on the site you have to buy coke and enter the codes on to the website. By doing the survey you don't have to spend a penny, and 2 for 1 vouchers seem to be hard to come by this year!