If the smart tag is showing correctly, but the link for enabling the RadCaptcha's HttpHandler is missing, it means that the handler is already registered in the web.config file (i.e. no need to register it), and the CaptchaImage will be shown on the control.
We have received requests from other customers to update our documentation, so it is clear what happens if the link is missing in the smart tag. We will update the documentation as soon as we can.
All the best,
the Telerik team