Hi Raindeer,
This is how you register the control in v4.master page (at the top of the page section where you see similar entries)
<%@ Register TagPrefix="Navigation" Namespace="WA.Navigation" Assembly="WA.Navigation, Version=4.0.0.0, Culture=neutral, PublicKeyToken=2b4b09f1c57c8f0b"%>
And this is how you add the web part (framgment of the XML web part definition)
<Navigation:PanelMenu runat="server" __MarkupType="xmlmarkup" WebPart="true" __WebPartId="{240BB0EE-8A42-433A-995C-6F5054006142}">
….