Have you added the Ajax toolkit to your page?I mean the dll! :) TextBoxWatermarkExtender AJAX extender can be attached to an ASP.NET TextBox control to get "watermark" behavior. When a watermarked TextBox is empty, it displays a message to the user with a custom CSS style(i.e your css file)The asp:TextBoxWatermarkExtender has the following main properties given below:
TargetControlID - The ID of the TextBox(name of the textbox where you want the watermark) to operate on.
WatermarkText - The text to show when the control has no value.(default text as watermark)
WatermarkCssClass - The CSS class to apply to the TextBox when it has no value.
<pre lang="vb"><ajaxToolkit:TextBoxWatermarkExtender ID="TBWE2" runat="server"
TargetControlID="TextBox Name"
WatermarkText="Watermark text"
WatermarkCssClass="watermarked" />