I am novice in CS extension development and couple of days back started digging into it.
I'm developing CS extension application for InDesign using Flex Builder 4 on Mac.
Can someone show me the right path to get the document and InDesing API/Library for Flex?
For InDesign API documentation you can refer to http://www.adobe.com/devnet/indesign/sdk.html. Their SDK includes API documentation and guides for developing InDesign plugins. For further information relating to the CS SDK then you can refer to the following devnet sites for links to tutorials, videos and API documentation:
Are you using CS Extension Builder?