The code should look something like:
<asp:TextBox ID="ContentMail" runat="server" AutoFocus="true" Width="60em" Height="40em" TextMode="MultiLine"/>
<asp:HtmlEditorExtender ID="HtmlEditorExtender1" runat="server" TargetControlID="ContentMail" DisplaySourceTab="True">
</asp:HtmlEditorExtender>
And if you are setting it in a TemplateField just add an invisible div with a textbox linked with a HtmlEditorExtender