implementing admob banners in unity 3d visual studio project

Hi everyone, first i admit that i don't really Know how to use visual studio express For Windows phone.

I have a compiled project from unity 3d and i would implement admob banners only in selected Scenes like some menues but not in game scenes.

I tryed to follow the tutorial on the admob page and to insert them in the main page.xmal but it doesn't work.

Where and How can i insert banners in the game? In the project i have a folder full of levels but i can't open them to edit and insert banners.