How can I enable the Creative Commons license for a video?
Make sure you are allowed to mark it as CC.
Things you should know
The ability to mark uploaded videos with a Creative Commons license is only available to users whose account is in good standing. You may check the status of your account under the Manage Account section of your Account Settings.
From here: http://www.youtube.com/t/creative_commons
You cannot mark your video with the Creative Commons license if there is a claim on it.