<Window x:Class="MyNamespace.MainWindow"
        xmlns="http://schemas.microsoft.com/winfx/2006/xaml/presentation"
        xmlns:x="http://schemas.microsoft.com/winfx/2006/xaml"
        xmlns:viewer="clr-namespace:GrapeCity.ActiveReports.Viewer.Wpf;assembly=GrapeCity.ActiveReports.Viewer.Wpf"
        Title="MainWindow" Height="450" Width="800">
    <Grid>
        <viewer:Viewer x:Name="reportViewer" />
    </Grid>
</Window>

public partial class MainWindow : Window
{
    public MainWindow()
    {
        InitializeComponent();
        reportViewer.LoadDocument("path/to/your/report.rdlx");
    }
}