struct MyApp : TopWindow {

    
    SplitterFrame sf, RightSide;	
	TreeCtrl   tree;
		
	typedef App CLASSNAME;  // <-- There is no type "App", it must be "MyApp"
...

	void ItemDC()
	{
		PromptOK(DeQtf("Clicked on line " + IntStr(tree.GetCursorLine()) + "!"));
	}