I've come to know that there's nothing similar like Activities in Marmalade. Because Marmalade works on Game loop, in which a particular rendering has to be done frequently in the loop, so as to make it look persistent on the screen. Something like what we do in the custom View's onDraw(Canvas canvas) method, although we use invalidate() method to do that instead of the loop.
The CIwMaterial looks similar like a fullScreen View which can be used in lace of the Activity, though there's nothing like the same in it. IwGxSetMaterial() is the global function to render the newer material on the screen.
This answer is not perfect, but atleast will help those who want to code in C++ using marmalade for the first time and previously doing it using the Android SDK.
PS: Kindly correct me wherever I am wrong.