It turns out this is a lot easier than I thought.
Click the Help menu item. Click the Manage AIR SDK item. When the screen opens, click the + sign and navigate to the folder where you extracted the SDK to.
That's it! Now you should be able to pick this SDK from the list of options in Flash.
Here's the link I used. The bottom section is about the newer version of Flash. http://code.tutsplus.com/tutorials/enable-the-latest-air-sdk-in-flash-professional-cs55--active-11488