For those didn't work :
1. First login/register to callofduty . c o m.
2. Link your steam account when logged in your web profile setting
3. Copy the link h t t p s : // profile . callofduty . com / dayzero and paste it in a new window.
4. Check the box "Please click here if you are entitled to this content and did not receive it."
5. Clik Submit