So sorry you are having problems entering. I just tried and had no problems. All you have to do is tweet about someone you are missing. You will hear the song and be automatically entered into the contest.
Instructions from Chris' staff:
Promos will direct people to Chris' website. The landing page will have the song streaming and details of the contest. People will have to 1) submit their email AND 2) Press the button to post a tweet.
Once completed, they will be entered to win... and can download the song for free.
President & CEO Issa Trust Foundaiton