There are two things you need to do before you can order from a giveaway. They are:
1) Set up your character and town information and 2) Add your friend code to ACC
How to set up your town and character
Step 1) Go to the left of your screen and under "My ACC" click on "My Towns"
Step 2) Go to "Add New Town" at the top right corner and click on it.
Step 3) Type your town name and select the game your town is in
Step 4) Select the information that applies to your town and hit "Save Town"
Step 5) Go to the left of your screen and under "My ACC" click on "My Characters"
Step 6) Select "Add new Character" on the upper right
Step 7) Fill in the information that applies to your character and hit "Save Character"
& You are done! Congratulations!
How to add your Friend Code
Step 1) Go to the left of your screen and under "My ACC" click on "My Friend Codes"
Step 2) At the top of the page you will see "My WiFi Games and Settings" click "Edit"
Step 3) Go to the Animal Crossing game you have. This is what you will be looking for:
New Leaf: Nintendo 3DS code
Step 4) Type your friend code and select your character if applicable.
Step 5) Go to the bottom and hit "Submit"
& You are done! Congratulations!