You can dynamically get the AppUi class of your application is using CEikonEnv function AppUi(). Cast return value to your specific AppUi-type, and you are ready to access functions in your AppUi-class. See CEikonEnv and CCoeEnv-class documentation in SDK help.
More straightforward way is to store a pointer (or reference for that matter) to your AppUi-class as a member variable and set that in your container/view/whatever class constructor like this:
Appui ConstructL code:
iMyView = new CMyView(<other arguments> ,this);