A basic example of how to show a dialog box using prism and the mvvm pattern in wpf

ToolbarA.xaml

<UserControl x:Class="Example.ToolbarA"
             xmlns="http://schemas.microsoft.com/winfx/2006/xaml/presentation"
             xmlns:x="http://schemas.microsoft.com/winfx/2006/xaml"
             xmlns:mc="http://schemas.openxmlformats.org/markup-compatibility/2006" 
             xmlns:d="http://schemas.microsoft.com/expression/blend/2008"
             xmlns:i="http://schemas.microsoft.com/expression/2010/interactivity"
             xmlns:interactionRequest="http://www.codeplex.com/prism"
             mc:Ignorable="d" 
             d:DesignHeight="300" 
             d:DesignWidth="300">

    <Grid>
        <i:Interaction.Triggers>
            <interactionRequest:InteractionRequestTrigger SourceObject="{Binding NotificationRequest, Mode=OneWay}">
                <interactionRequest:PopupWindowAction IsModal="True" CenterOverAssociatedObject="True"/>
            </interactionRequest:InteractionRequestTrigger>
        </i:Interaction.Triggers>

        <Button Command="{Binding Path=RaiseNotificationCommand}">Click Me</Button>
    </Grid>
</UserControl>

ToolbarAViewModel.cs

using System;
using Microsoft.Practices.Prism.Commands;
using Microsoft.Practices.Prism.Interactivity.InteractionRequest;

namespace Example
{
    public class ToolbarAViewModel
    {
        public ToolbarAViewModel()
        {
            RaiseNotificationCommand = new DelegateCommand(RaiseNotification);
            NotificationRequest = new InteractionRequest<INotification>();
        }

        public DelegateCommand RaiseNotificationCommand { get; private set; }
        public InteractionRequest<INotification> NotificationRequest { get; private set; }
        public string InteractionResultMessage { get; set; }

        private void RaiseNotification()
        {
            var notification = new Notification
            {
                Content = "Notification Message",
                Title = "Notification"
            };

            Action<INotification> lambda = _ =>
            {
                InteractionResultMessage = "The user was notified.";
            };

            NotificationRequest.Raise(notification, lambda);
        }
    }
}