For anyone still running into this issue, we just figured out a workaround for debugging on Mac. There were two problems:
1) The precompiled ANE we were using - v0.7 from https://github.com/Ventero/FRESteamWorks - wasn't fully updated for 64-bit Mac. The code was correct, it was just the downloadable build (http://dump.ventero.de/FRESteamWorks) that was out of date. We needed to recompile the ANE ourselves to avoid always using the "default" implementation of the ANE.
2) This fixed our packaged release build, but when trying to run in the debugger, we weren't able to correctly set the DYLD_FALLBACK_LIBRARY_PATH environment variable before Flash Builder invoked the adl process. Since this is a required step for finding the dynamic Steam libraries, we needed to put the Steam libraries in one of the default Mac locations for dynamic libraries:
This allowed us to run the debugger on the process without getting the dreaded "ArgumentError: Error #3500: The extension context does not have a method with the name AIRSteam_Init."
FYI, I'd much prefer to use DYLD_FALLBACK_LIBRARY_PATH, but it appears that DYLD_FALLBACK_LIBRARY_PATH is a protected variable on MacOS (as of El Capitan) and gets purged frequently, so it needs to be overridden manually whenever you need it (https://apple.stackexchange.com/questions/212945/unable-to-set-dyld-fallback-library-path-in-shell-on-osx-10-11-1).
Ideally the documentation for FRESteamWorks could be updated with this important step, along with a correctly recompiled version with 64-bit Mac support.
Hope this helps!