Extension builder 2.1 is not a part of CC, it has be purchased separately.
Adobe Extension Builder 2.1 is a powerful Eclipse™ based development tool that provides a simple way to develop, debug, and package native and Adobe Flash® technology–based extensions that run inside and integrate with Creative Suite and Creative Cloud applications.
It is listed in: http://www.adobe.com/products/adobe-extension-builder.html
And also need Flahs builder premium to work with Extension Builder? or not?