The alert will never display because of the Response.Redirect call.
To make this work, use window.location in JavaScript to redirect the page after the alert and comment out the call to Response.Redirect.
Dim strmessageEdit As String
strmessageEdit = "Record Deleted Successfully"
Dim strScriptEdit As String = _
"<script runat=""server"" language=""JavaScript"">alert('" & _
strmessageEdit & _
"'); window.location = '" &
Page.ResolveUrl("~/Add.aspx") & _
"';</script>"
Page.RegisterStartupScript("clientScript", strScriptEdit)