I am trying to use the following code from your demo to try and use the windows control:

function openRadWin() {
        radopen("http://www.telerik.com", "RadWindow1");
    }

<telerik:RadWindowManager ID="RadWindowManager1" runat="server" EnableShadow="true">
    <Windows>
        <telerik:RadWindow ID="RadWindow1" runat="server" ShowContentDuringLoad="false" Width="400px"
            Height="400px" Title="Telerik RadWindow" Behaviors="Default">
        </telerik:RadWindow>
    </Windows>
</telerik:RadWindowManager>

<button style="width: 150px; margin-bottom: 3px;" onclick="openRadWin(); return false;">open RadWindow</button>

I get an error, screen capture attached.