Could you please bi a little bit more specific on what exactly do you want to change on the ManipulationAdorner? If you want to change it's appearance you'll have to edit the default template of the RadDiagram and set a style to the ManipulationAdorner part.
I've attached a sample project so could you please examine it and if you have further questions feel free to ask.
All the best,
the Telerik team