the Flash runtime only supports draft-ietf-behave-turn-8 (with no authentication) for RTMFP, not any later draft or the final RFC version:
i don't know where you could get something like that now, unfortunately. i wrote the one we use here.
if you happened to have such a TURN proxy server, you configure Flash Player to use it by adding a line to your mms.cfg file (location depends on your operating system; on the Mac it's at /Library/Application Support/Macromedia/mms.cfg). you add a line that says: