Activation error with IEventAggregator Prism

Question

I am using Prism in my WPF. When I add IEventAggregator as a parameter to the ViewModel constructor I get this error: An exception of type 'Microsoft.Practices.ServiceLocation.ActivationException' occurred in Microsoft.Practices.ServiceLocation.dll. Additional information: Activation error occurred while trying to get instance of type Object, key "CategoriesView"

The exception is triggered in this line:

private void NavigateToCategoriesRadioButton_Click(object sender, RoutedEventArgs e)
{
    this.regionManager.RequestNavigate(RegionNames.ConfigurationContentRegion, categoriesViewUri);
}

where categoriesViewUri is:

private static Uri categoriesViewUri = new Uri("/CategoriesView", UriKind.Relative);

This is my view model class:

[Export]
public class CategoriesViewModel : BindableBase
{
    private readonly IRegionManager regionManager;
    private readonly IEventAggregator eventAggregator;
    private readonly IConfigurationCategoriesService categoriesService;
    private readonly ObservableCollection<Category> categoriesCollection;
    private readonly ICollectionView categoriesView;
    private readonly DelegateCommand<object> deleteCategoryCommand;

    [ImportingConstructor]
    public CategoriesViewModel(IEventAggregator eventAggregator, IConfigurationCategoriesService categoriesService, IRegionManager regionManager)
    {    
        this.categoriesService = categoriesService;
        this.regionManager = regionManager;
        this.eventAggregator = eventAggregator;

        this.deleteCategoryCommand = new DelegateCommand<object>(this.DeleteCategory, this.CanDeleteCategory);

        this.categoriesCollection = new ObservableCollection<Category>(categoriesService.GetCategories());
        this.categoriesView = new ListCollectionView(this.categoriesCollection);
        this.categoriesView.CurrentChanged += (s, e) => this.deleteCategoryCommand.RaiseCanExecuteChanged();

    }

    public ICollectionView Categories
    {
        get { return this.categoriesView; } 
    }

    public ICommand DeleteCategoryCommand
    {
        get { return this.deleteCategoryCommand; }
    }

    private void DeleteCategory(object ignored)
    {
        var category = this.categoriesView.CurrentItem as Category;
        if (category != null)
        {
            categoriesService.DeleteCategory(category);
        }
    }
    private bool CanDeleteCategory(object ignored)
    {
        return true;
    }
}

It looks like CatagoriesViewModel cannot get an instance of IEventAggregator on the constructor but this is done automatically by Prism, isn't it? In the example I have from Prism Documentation (StockTraderRI_Desktop) I don´t see anywhere where the EventAggregator is instantiated. Can anyone see what am I getting wrong? Thanks in advance

Editted:

The Navitagion item view is registerd in the CategoriesModule class:

[ModuleExport(typeof(CategoriesModule))]
public class CategoriesModule : IModule
{
    [Import]
    public IRegionManager regionManager;

    public void Initialize()
    {
        this.regionManager.RegisterViewWithRegion(RegionNames.ConfigurationNavigationRegion, typeof(CategoriesNavigationItemView));
    }
}  

And CategoriesView code-behind is:

[Export("CategoriesView")]
public partial class CategoriesView : UserControl
{
    public CategoriesView()
    {
        InitializeComponent();
    }

    [Import]
    public IRegionManager regionManager;

    [Import]
    public CategoriesViewModel ViewModel
    {
        get { return this.DataContext as CategoriesViewModel; }
        set { this.DataContext = value; }
    }
}

Answer 1

I solved this issue adding the following using statement:

using Microsoft.Practices.Prism.PubSubEvents;

instead of

using Prism.Events;

I also switched to Unity instead of MEF by recommendation on this site.