Edit Accordion(AjaxToolkit) in Design View

I had the same problem. I just now the following steps:

1) Create a test page.
2) Create the layout on the test page.
3) Copy the entire code and paste it inside the Content
4) Done!

Or a better option would be:

1) Create a CustomControl.
2) Create the layout on the CustomControl.
3) Put the CustomControl inside the Content
4) Done!