captcha in asp.net

implement captcha in form, i am trying to implement captcha in this way

<%@ Register Assembly="MSCaptcha" Namespace="MSCaptcha" TagPrefix="cc1" %>
  
  
  
      
    
  
      
     
         

protected void btnVerify_Click(object sender, EventArgs e)
{
    cptCaptcha.ValidateCaptcha(txtCaptcha.Text.Trim());
    if (cptCaptcha.UserValidated)
    {
        lblErrorMessage.ForeColor = System.Drawing.Color.Green;
        lblErrorMessage.Text = "Valid text";
    }
    else
    {
        lblErrorMessage.ForeColor = System.Drawing.Color.Red;
        lblErrorMessage.Text = "InValid Text";
    }
}

but when i use handlers in webconfig

got an exception my application not run