Add the following javascript function to the onclick event from the linkbutton in the grid
eg
onclick="showModalPopup(); return false"
The "return false" allows the linkbutton not to post back
<script type="text/javascript">
function showModalPopup() {
var modalPopupBehavior = $find('mdlPopup');
modalPopupBehavior.show();
}
</script>
[edit] pre tags added [/edit]