To overlay Adobe AIR 3.7 on top of a copy of Flex 4.6 SDK:
1. Download version 3.7 of the Adobe AIR SDK for Flex Users (non compiler version underneath the "Download Now" buttons).
2. Create a copy of the 4.6.0 sdk folder in "/Applications/Adobe Flash Builder 4.6/sdks/"
3. Copy the downloaded SDK archive file, AdobeAIRSDK.tbz2, to the newly created folder, for example "/Applications/Adobe Flash Builder 4.6/sdks/4.6.0-AIR3.7"
4. Unpack the archive within the new 4.6.0-AIR3.7 folder by running $ tar xvf AdobeAIRSDK.tbz2
5. Launch Flex Builder, under Project > Properties > Flex Compiler click Configure Flex SDK and add a link to the new folder to add the new SDK.
6. You should now be able to select which SDK in the dropdown -- Flex 4.6.0 (build 23201) AIR 3.7
7. Update the AIR Application Descriptor file with the new namespace xmlns="http://ns.adobe.com/air/application/3.7"