based on: http://mattberseth.com/blog/2007/10/modalpopup_with_multiple_cance.html
1. Add BehaviorID to the Popupextender:
<cc1:ModalPopupExtender BehaviorID="mdlPop" ..
(from Ajax.asp.net: BehaviorID: In cases where you would like to access the client-s... [More]