In order for this to work, the object you are trying to call from Flash needs to have global scope. I.e., this will not work:
// closure to keep vars out of global scope - generally a good thing!
(function() {
var c;
function MyClass() {
}
MyClass.prototype.myFunction = function() {
alert('Do something!');
}
c = new MyClass();
})();
... meanwhile, in Flash ...
ExternalInterface.call("c.myFunction");
What you need is a global entry-point for the object. What errors are you getting, though? Are you getting null back from the call? Is any specific Error being thrown?