The purpose of <Any>Observer classes is to define an interface.
You should implement the class which has this interface and place implementation there. Later e.g. symbian code is able to call your method. In other word you can not found a class of MsSessionObserver, instead you must implement it yourself...great ?
An example:
[symbian code]:
class MHelloWorldObserver
{
virtual void ShowhelloMessage(TDesC aMess) = 0;
};