Modal example from tutorial website not working 1:1 copy

HI,

I followed this tutorial.
1:1 I copied it into my project.
But the modal does not open.
If I remove the display:none (then it shows) but using the
"close" button. It does not close.

Why is that?

http://www.aspsnippets.com/Articles/Building-Modal-Popup-using-ASPNet-AJAX-ModalPopupExtender-Control.aspx