Pull to refresh On Windows Phone

I am trying to implement a pull to refresh on my windows phone application,

I tried this sample : http://code.msdn.microsoft.com/wpapps/TwitterSearch-Windows-b7fc4e5e but when I want to compile the project I have this error :

TwitterViewModel does not exist in the namespace "clr-namespace:TwitterSample.ViewModels"

But the assembly is properly referenced in the xaml :

xmlns:vm="clr-namespace:TwitterSample.ViewModels"

How can I correct this?