Sorry for replying so late to your question, I'd missed it as a comment.
In your code here, you're doing an import of a IWordProcessor onto your WordProcessor member variable. You're also (in the same class) initializing the composition container with all the Assemblies that have shipped with your Silverlight XAP.
By the way - you should *not* have to write that code because it is the default behaviour of MEF in Silverlight anyway so you should be able to get rid of all the code in the function InitializeContainer().
However, if you want MEF to satisfy the import that you have on your MainPage then you do need to call CompositionInitializer.SatisfyImports() on that instance so you can't comment that code out.
Hope that helps.