Hi,
You are confusing about HTML and ASPX. "asp:Button" is ASPX syntax, so you can not use it at places only simple HTML is allowed, such as HeaderHtml property. There is no such syntax to include a server control as a attribute value (inside quotation marks).
In order to have any button in the dialog header, you must use plain HTML. For example:
Code: HTML/ASPX
<input type="button" value="Button1" onclick="window.alert('hi');" />
FooterArea is different. You can edit the dialog's FooterTemplate and put whatever server controls in it. The difference is, FooterTemplate is an ITemplate, so it does take server controls. HeaderHtml is a string. So it only takes simple HTML.
Hope this helps.
Thanks!