1.The
tagprefix
used by you in "@Register" directive should not be "asp", because "asp" is reserved for system ASPX controls.
2.It seems that you forgot to provide all ASPX code, because your "ModalPopupExtender" control is not closed properly.
3.You can find example about the correct using of the Modal Popup Extender in my next article:
Advanced ASPX GridView Pagination and Data Entities[
^]