Popup new page on Gridview column click

Please see check this link :

http://www.dotnetpools.com/Article/ArticleDetiail/?articleId=335&title=Code-To-Display-Gridview-Selected-Row-Value-in-Modalpopupextender-Using-C#-in-Asp.net

I copied all the code but i am getting this error on compilation:
The type or namespace name 'ModalPopupExtender' does not exist in the namespace 'System.Web.UI.WebControls' (are you missing an assembly reference?)

Please help. The code underlines the name "ModalPopupExtender" with green and says its not known element