Hello rent,
You use a Dock Panel of course. Unfortunately Silverlight does not have one - WPF does. In the Telerik.Windows.Control.Primitives there is a RadDockPanel that behaves like the WPF one and you can use it like this:
Eventually you can use a simple Grid Panel here but on each menu item you will have to set Grid.Column and add ColumnDefinition on the Grid for each RadMenuItem which is not very convenient since if you reorder the items you will have to reset these values.
Kind regards,
Panayot
the Telerik team