Jquery – mvc3 – ajax form submit and server side validation

I am sure this has been asked already, but I have been trying all the solutions given, but none of the given solution is solving my problem. Probably, I am not implementing it right.

I am using MVC3, razor, jQuery.

I have a new user registration form. Once registration is successful, I want to display the successful message (different on type of registration) in a dialog box.

If there is any error with ModelState, means ModelState.IsValid == false, I want to display errors in ValidationSummary.

View

function OnBegin () { alert('begin'); return $('form').validate().form(); }
function OnSuccess () { alert('success'); DisplaySucess()}
function OnFailure () { alert('failure'); }
function DisplaySuccess() {
    var typeOfRegistration = $('input:radio[name=RegistrationType]:checked').val();
    var msg;
    if (typeOfRegistration == 0) {
        msg = "You are successfully enrolled for Algebra classes.";
    }
    if (typeOfRegistration == 1) {
        msg = "You are registered with your email address. Someone would contact you with an email.";
    }
    if (typeOfRegistration == 2) {
        msg = "Thank you for filling up this form. One welcome packet would be delivered with in next 15 days";
    }

    msg = msg.replace(/\n/g, '<br />');
    $('#dialog').html(msg);
    $('#dialog').dialog('open');
}

@model Models.RegistrationModel
@using (Ajax.BeginForm("NewRegistration", "Account", new AjaxOptions() { OnBegin="OnBegin", OnSuccess = "OnSuccess", OnFailure = "OnFailure" }))
{ 
  @Html.ValidationSummary()
    <div>
    Register<br />
    @Html.RadioButton("RegistrationType ", "0", true) New Algebra class<br />
    @Html.RadioButton("RegistrationType ", "1") New user registration by email<br />
    @Html.RadioButton("RegistrationType ", "2") New user registration by mailing address<br/>
   ..some more types
</div>
<div>
    …here I display/hide controls based on Registration type selected.
        <div>
    @Html.LabelFor("UserName")
    @Html.TextBox("UserName")</div>
}

Controller

public ActionResult NewRegistration(RegistrationModel model)
{

         if (Request.IsAjaxRequest())
         {
             if (!ModelState.IsValid) { 
        //Ques: How do I display this in ValidationSummary. 
        return Json(new object[] { false, "Contains Email Address", ModelState.GetAllErrors() }); 
        }
             else { 
        if (UserNameIsTaken){ 
            //Ques: This does not show in ValidationSummary
            ModelState.AddModelError("_FORM", "This username is already taken. Please pick a different username.");
        }
       }
    }
         else if (!ModelState.IsValid) { return View(model); }

}

Model

public class RegistrationModel: IValidatableObject
{
[Required] //this gets displayed in ValidationSummary as Client-side validation
    public string UserName { get; set; }
    .. and all fields here
}
public IEnumerable<ValidationResult> Validate(ValidationContext context)
{
        if (!string.IsNullOrEmpty(UserName) && UserName.Contains(EmailAddress)) {
            yield return new ValidationResult("Username cannot contain email address", new[] { "UserName" });
        }
    }

Web.config has

UnobtrusiveJavaScriptEnabled="true" and ClientValidationEnabled="true"

.js files included are

script src="@Url.Content("~/Scripts/jquery-1.7.1.min.js")" type="text/javascript">

script src="@Url.Content("~/Scripts/jquery.validate.min.js")" type="text/javascript">

script src="@Url.Content("~/Scripts/jquery.validate.unobtrusive.min.js")" type="text/javascript">

script src="@Url.Content("~/Scripts/jquery.unobtrusive-ajax.min.js")" type="text/javascript">

What am I missing here? Any alternative?
Thanks in advance.