<telerik:RadStyleSheetManager runat="server"></telerik:RadStyleSheetManager>

<asp:UpdatePanel runat="server">
    <ContentTemplate>
        <asp:Button Text="Postback" OnClick="Unnamed_Click" runat="server" />
        <telerik:RadButton runat="server" Visible="false" ID="RadButton1" Text="Telerik button" AutoPostBack="true" />
    </ContentTemplate>
</asp:UpdatePanel>

protected void Unnamed_Click(object sender, EventArgs e)
{
    RadButton1.Visible = true;
}

<add key="Telerik.ScriptManager.EnableHandlerEncryption" value="true" />

<add key="Telerik.ScriptManager.EnableHandlerEncryption" value="false" />