Get access to the MEF catalog

   As with every dependency, you just need to make the catalog available through MEF so you can inject it where you need it. Here's how you can accomplish this.
    public class AppBootstrapper : CocktailMefBootstrapper<MainViewModel>
    {
        private ComposablePartCatalog _catalog;

        protected override ComposablePartCatalog PrepareCompositionCatalog()
        {
            // Hang on to the catalog
            return _catalog = base.PrepareCompositionCatalog();
        }

        protected override void PrepareCompositionContainer(CompositionBatch batch)
        {
            base.PrepareCompositionContainer(batch);

            // Add catalog to the MEF container
            batch.AddExportedValue(_catalog);
        }
    }

    [Export]
    public class MainViewModel : Screen
    {
        [ImportingConstructor]
        public MainViewModel(ComposablePartCatalog catalog)
        {

        }
    }