I figured it out.
IE does not support ".target". So I add checked before my code.
scheduler.attachEvent("onEmptyClick", function (date, native_event_object){
if(typeof native_event_object.target === 'undefined')
var currTarget=native_event_object.srcElement;
else
var currTarget=native_event_object.target;
if(!$(currTarget).hasClass('dhx_scale_holder')){
var s=confirm("Are you sure, You want to take an appointment");
if(s){ // proceed appintment}
}
});