<Button x:Name="Login" Click="Login_Click" Canvas.Left="200" Canvas.Top="360" Content="Login" FontSize="20" Height="40" Width="80"/>

<ListBox x:Name="DataBox1" Canvas.Left="330" Canvas.Top="332" Height="93" Width="130" />

public IQueryable<String> forenameOne;

public void Login()
{
    //var conductor = this.Parent as IConductor;
    //conductor.ActivateItem(new AdministrationViewModel());

    using (TableClassesDataContext db = new TableClassesDataContext())
    {
        var userdetails = from v in db.GetTable<PlannerUser>()
                          select v.Forename;

        MessageBox.Show("User details: " + userdetails.ToString());
        DataBox1 = userdetails;

    }

}

DataBox1.ItemsSource = userdetails;