R – XML to JSON in Flex

You need to download, install, and reference the utility library that contains the JSON routines. See http://labs.macromedia.com/wiki/index.php/ActionScript_3:resources:apis:libraries#corelib

Did you do that, and include it in your project?

This may be what you're looking for :

http://code.google.com/p/flit/

I haven't tested it yet so I can't really say if it's worth it or not