The client-side API of RadDock from the last year is a little bit changed to follow the ASP.NET Ajax naming conventions. You can use the RadDock Client Object help article
for reference. That is why, the line you mention fails. You need to replace get_DockZoneID()
var zone2 = $find(dock.get_DockZoneID());
For your convenience I have attached a modified version of the example from the Make each dockzone just hold one dock?
forum thread which uses the new client-side API of the control.
the Telerik team