Hi JC Wu,
Have you tried with:
ajaxManager = $find(
"<%= RadAjaxManager.GetCurrent(Page).ClientID %>"
In a DNN module, you may not (and should not) have RadAjaxManager placed in the module. It needs to go in a master page or an ASPX page if no master. Therefore, we use the static RadAjaxManager.GetCurrent()
method to get a reference to the current instance of the manager.
the Telerik team