>When you are referencing an object from your main menu to your forms, using, can references a method or object that you have create. If so how ?

You should provide reference link when you create an object, e.g.
oApp.MyObject=CreateObject(...)
and later on you can call it using
oApp.MyObject.MyMethod()
Note, that usually you don't need to put everything into application object properties, it's better to have some king of object hierarchy (oApp points to some modules which linked to some forms, which linked to some objects etc.)