Java.Lang.ClassNotFoundException. Xamarin Android Binding Project

I created a Xamarin Android Binding Project which uses a jar library (Vufori.jar) then i build project and a .dll file is created.
Then I added to my Solution a new android project. i added the .dll file to my references.

Now i can use all class from the .dll and when i build solution, i don't have problems.

But when i start debug, my app crash and i recive an Exception :
"Java.Lang.ClassNotFoundException: com.vuforia.Vuforia" at this line :
"return JNIEnv.FindClass ("com/vuforia/Vuforia", ref java_class_handle);"
Can you help me?
Thanks