Basically, it's the same way that everyone used to do it with the Adobe Flex SDK:
When you download the AIR SDK, make sure that you don't download the AIR SDK & Compiler. You need to get the version that's designed for Flex. On the AIR SDK download page, there's a section near the bottom that starts with this text:
Note: Flex users will need to download the original AIR SDK without the new compiler.
The correct downloads appear below this message.