I managed to find out what was the problem in setting up the framework.
I was targeting WPF not Silverlight. there is a little difference in the code.
The App.xaml should be look like this:
<local:HelloBootstrapper x:Key="bootstrapper" />
The difference is that we should use MergedDictionaries to setup the Bootstrapper. and we don't need to have any thing in the App.xaml.cs.