I have tested ken's code, it works.
<%@ Page Language="C#" ValidateRequest="false" %>
<%@ Register TagPrefix="CE" Namespace="CuteEditor" Assembly="CuteEditor" %>
<html>
<head>
</head>
<body>
<form id="Form1" runat="server">
<CE:Editor ID="editor1" runat="server">
</CE:Editor>
<asp:Button ID="button1" runat="server" Text="Open Template" OnClientClick="openTemplate();return false" /> </form>
</body>
</html>
<script>
var editor1 = document.getElementById("<%= editor1.ClientID %>");
function openTemplate()
{
editor1.ExecCommand("InsertTemplate");
}
</script>
Please check whether the value of ID is same as the value of script
<CE:Editor ID="editor1" runat="server">
</CE:Editor>
Regards,
Eric