In the Terms of Service, it says:
5.2 Scratch provides support materials, including images, sounds, video, and sample code, to help users build projects. Support materials are licensed under the Creative Commons Attribution-ShareAlike 2.0 license. You may also use screenshots of Scratch under the same license. Please note that this does not apply to materials that are also trademarked by the Scratch Team or other parties as described in parts 5.4 and 5.5, below.
The Creative Commons Attribution-ShareAlike 2.0 license requires you to attribute any material you use to the original author. When you use Scratch support materials, or screenshots of the Scratch website, please use the following attribution: "Scratch is developed by the Lifelong Kindergarten Group at the MIT Media Lab. See http://scratch.mit.edu."