I have a problem with ModelField

[HenrySseki]

I am new to Ext.Net. I setup everything as the guide but I created a view, added a sample MVC code sample and I get an error of
'The type or namespace 'ModelField' could not be found(are you missing a using directive or an assembly reference?)'.

@using Ext.Net.HtmlHelpers
@using Ext.Net.Core

@section headtag{

}
@section home
{
    <h1>Simple Array Grid</h1>

    @(Html.X().GridPanel()
       .Title("Cell commands")
       .Width(700)
       .Height(300)
       .Store(Html.X().Store()
            .Model(Html.X().Model()
                .Fields(
                     new ModelField("company"),
                     new ModelField("price", ModelFieldType.Float),
                     new ModelField("change", ModelFieldType.Float),
                     new ModelField("pctChange", ModelFieldType.Float)
                     )
                )
            .DataSource(Model)
            )
        .ColumnModel(
            Html.X().Column().Text("Company").DataIndex("company").Flex(1),
            Html.X().Column().Text("Price").DataIndex("price"),
            Html.X().Column().Text("Change").DataIndex("change").Renderer("change"),
            Html.X().Column().Text("Change").DataIndex("pctChange").Renderer("pctChange"),
            Html.X().DateColumn().Text("Last Updated").DataIndex("lastChange")
        )
    )
}