MVC :: Error "CS1928: 'System.Web.Mvc.HtmlHelper<DataLayer.BlogPost>' Does Not Contain A Definition For CheckBoxList"

I'm using a html helper that i got from here: [URL] here is my modified version (using SelectListItem):

public static string CheckBoxList(this HtmlHelper htmlHelper, string name, List<SelectListItem> listInfo)
{
return htmlHelper.CheckBoxList(name, listInfo, ((IDictionary<string, object>)null));
}
public static string CheckBoxList(this HtmlHelper htmlHelper, string name, List<SelectListItem> listInfo, object htmlAttributes)
{
return htmlHelper.CheckBoxList(name, listInfo, ((IDictionary<string, object>)new RouteValueDictionary(htmlAttributes)));
}
public static string CheckBoxList(this HtmlHelper htmlHelper, string name, List<SelectListItem> listInfo, IDictionary<string, object> htmlAttributes)
{
if (String.IsNullOrEmpty(name))
throw new ArgumentException("The argument must have a value", "name");
if (listInfo == null)
throw new ArgumentNullException("listInfo");
if (listInfo.Count < 1)
throw new ArgumentException("The list must contain at least one value", "listInfo");
StringBuilder sb = new StringBuilder();................

I can't seem to figure out why this always throws an error: CS1928: 'System.Web.Mvc.HtmlHelper<DataLayer.BlogPost>' does not contain a definition for 'CheckBoxList' and the best extension method overload 'HtmlHelpers.HtmlHelpers.CheckBoxList(System.Web.Mvc.HtmlHelper, string, System.Collections.Generic.List<System.Web.Mvc.SelectListItem>)' has some invalid arguments