New Post: ResizingPanel Not Found

I downloaded the latest WPF Toolkit, unblocked the DLLs and added them to a new WPF .NET 4.0 project. Here is my Main Window XAML:
<Window x:Class="Avalon.MainWindow"

    xmlns="http://schemas.microsoft.com/winfx/2006/xaml/presentation"
    xmlns:x="http://schemas.microsoft.com/winfx/2006/xaml"
    xmlns:xctk="http://schemas.xceed.com/wpf/xaml/toolkit"
    xmlns:xcad="http://schemas.xceed.com/wpf/xaml/avalondock"

The following line works fine:
<xcad:DockingManager x:Name="_DockManager">

But this line will give ma an error:
<xcad:ResizingPanel>

It also fails if I try to create the ResizingPanel, or any panel, in code.

If I look at the AvalonDock in Object Browser, as well as the rest of Extended WPF toolkit, there is no Resizing Panel.

What is the trick to get this working?