1) Assign a BehaviourID to the ModalPopupExtender using the BehaviourID attribute. 

BehaviorID ="ModalBehaviour"

2) Use the $find method to get a handle to the Modal Popup Behaviour . 

$find ("ModalBehaviour").

3) Call your hide and show methods on the acquired handle.

4) The Javascript would look like this.

 

<script language="javascript"> 

function ShowModalPopup() 
{
  $find("ModalBehaviour").show(); 
}

function HideModalPopup() 
{
 $find("ModalBehaviour").hide(); 
}

</script>