This FB help topic might be of use:
Accessing Document and Application object scopes
In your application's main MXML file, the file that contains
the <mx:Application> tag, you can access the methods and
properties of the Application object using the this keyword.
However, in custom ActionScript and MXML components, event
listeners, or external ActionScript class files, Flex executes in
the context of those components and classes, and the this keyword
refers to the current Document object and not the Application
object. You cannot refer to a control or method in the application
from one of these child documents without specifying the location
of the parent document.
Flex provides the following properties that you can use to
access parent documents:
mx.core.Application.application The top-level Application
object, regardless of where in the document tree your object
mx.core.UIComponent.parentDocument The parent document of the
current document. You can use parentDocument.parentDocument to walk
up the tree of multiple documents.
mx.core.UIComponent.parentApplication The Application object
in which the current object exists. Flex applications can load
applications into applications, therefore, you can access the
immediate parent application by using this property. You can use
parentApplication.parentApplication to walk up the tree of multiple