We inspected the code and it seems that the problem is in the name of the event. It should be without the brackets at the end just like that: OnClientAppointmentClick="openwin"
Here is the code that you need for code behind implementation as well:
protected void RadScheduler1_AppointmentClick(object sender, SchedulerEventArgs e)
Details.VisibleOnPageLoad = true;
Hope this will help you.
the Telerik team