Unfortunately the only way to enable advanced Telemetry is for the SWF to have the right SWF tag - there's no way to turn it on/off from ActionScript. If you're having difficulty enabling it, Renaun made a handy AIR app that lets you add advanced Telemetry support to existing SWFs:
This will work regardless of how you compiled the SWF (i.e. it's fine for Flex apps too).
If you just want basic information (i.e. no ActionScript sampling) then you don't need to do this. You just need to setup the .telemetry.cfg file on your server to point to Scout. See here:
In fact, you might be able to enable telemetry in as3 As Michael said, you need the tag, but you can add it on the fly to your loaderInfo.bytes and load yourself as its own child - means you would also need a check if you're loader or loaded atm. This will not work if the player only cares about top-level movie tags, though.
Now I actually tried that and it does not work. Only top-level movie tags matter.