I see now that only ActionScript or ActionScript Mobile projects support this option:
Important: The advanced telemetry options are available only for ActionScript projects and ActionScript Mobile projects that use the ActionScript Compiler 2.0 (ASC 2.0).
That's a shame. Any reason Flex project's can't get the same support? Flex just compiles down to AS at the end of the day anyway, so why the limitation?
For Flex projects you can use the python script option mentioned at http://www.adobe.com/devnet/flashruntimes/articles/adobe-scout-getting-started.html#articl econtentAdobe_numberedheader_2 - The script will post process your swf to enable telemetry.
There is also an AIR app available to enable temetry on your swf. See http://renaun.com/blog/2012/12/enable-advanced-telemetry-on-flex-or-old-swfs-with-swf-scou nt-enabler/