The github twentytwelve is not the same as a manually downloaded theme. You will need to unzip it and look for the twentytwelve subfolder. The entire zip contains code snippets and files in different stages of developement from the currend 'build' folder for the theme. I actually grabbed my copy a while back while the them was still in the trunk of WordPress 3.4 beta. I really like it. I do custom themes for clients built on twentyten and twenty twelve. It's everything they said it would be. Modern, clean and responsive. The @media breakpoints for elements are built in-place. They aren't sectioned by screen width. So when you modify an element you will see the 600px version of the element just below it. No more jumping around or needing to design in stages for each width. And it looks like only one breakpoint at 600px right now. If you want to see what way going on and some idea as to why it was punted.. read here http://wpdevel.wordpress.com/tag/twentytwelve/