[dosya]

var flashvars = {params: {config:"blablalba.xml"}};
trace(flashvars);                      //[object Object]					
trace(flashvars['params']);        //[object Object]
trace(flashvars.params.config);  //blablalba.xml

А если уж совсем хочется с _root то

var flashvars = {params: {config:"blablalba.xml"}};
_root=flashvars;
trace(_root);                        //[object Object]					
trace(_root.params);             //[object Object]
trace(_root['params'].config);  //blablalba.xml