imageView => SetImageLevel how ?

Hello,
i'm a beginner with xamarin android (visual studio community 2015) and i've just to read this excellent article :
https://developer.xamarin.com/recipes/android/networking/gsm_strength/

Now, most of this article work fine just one thing not work => the image view is always null !
all right i think i must to put some image inside my app but i can't understand ...

i must to put image with name 0 to 100 inside my app ?
i can't understand how to realize => SetImageLevel

thanks for your time guys