In the development version, there's the property "stage" on the Starling object that gives you the stage; the first child of the stage will be your game class, and you can cast it to that. (The official 0.9 version doesn't contain that property yet.)
However, the object won't be available right away: Starling will create an instance of that class only after Stage3D is finished initializing. You'll find out when it's done by listening to the following event:
I hope that helps!