The following registration requires a Telerik.Web.UI dll
<%@ register tagprefix="telerik" namespace="Telerik.Web.UI" assembly="Telerik.Web.UI" %>
assembly contains all Telerik's controls from the RadControls for ASP.NET suite.
The examples we suggested you in our previous posts were related to RadDock for ASP.NET AJAX as this is a forum thread about RadDock for ASP.NET AJAX. Questions about RadDock for ASP.NET you can post in the corresponding forums.
Regarding your initial question for adding a link button in raddockable object title bar, the requested functionality is not supported by RadDock for ASP.NET. However, you can add custom commands and execute custom code when clicking on a custom command. An example about custom commands in RadDock for ASP.NET you can find here
All the best,
the Telerik team