I just added System.Windows.Interactivity assembly. XamlParse throw me an exception on run time:
Could not load file or assembly 'System.Windows.Interactivity,
PublicKeyToken=31bf3856ad364e35' or one of its dependencies. The
system cannot find the file specified.
Just a guess, might be you are not referencing the libraries in the MAIN project.
It has happened to me several times.