Adobe Flash isn't using Charles on Mac OS X
Unfortunately there is a problem with Flash on Mac OS X. It does not use the OS proxy settings and does not appear to offer any proxy settings of its own. Therefore when you run Flash movies in the authoring environment or stand-alone they do not use Charles.
However, if you run your Flash movie in a browser it will use the correct proxy settings and use Charles.
A possible workaround for this problem is to use the Reverse Proxy feature of Charles so that you can make the URL you connect to actually pass through Charles (eg. http://localhost:12345/).