It’s easy to make a Flex SDK from an AIR SDK for which you don’t have a corresponding Flex SDK. This is what you do:
1. Download AIR SDK.
2. Locate the existing Flex SDK.
For a default installation, it’s available in the C:\Program Files\Adobe\Adobe Flash Builder Version\sdks folder.
3. In this location, extract AIR SDK to a new folder .
4. Copy the contents of your existing Flex SDK and paste it to the folder where you have AIR SDK, WITHOUT over writing any of the files.
Flex SDK is ready. When you create a new project, use this as the SDK.