Here you go:

[HttpPost]
public ActionResult Upload(int? chunk, string name)
{
    var fileUpload = Request.Files[0];
    var uploadPath = Server.MapPath("~/App_Data");
    chunk = chunk ?? 0;
    using (var fs = new FileStream(Path.Combine(uploadPath, name), chunk == 0 ? FileMode.Create : FileMode.Append))
    {
        var buffer = new byte[fileUpload.InputStream.Length];
        fileUpload.InputStream.Read(buffer, 0, buffer.Length);
        fs.Write(buffer, 0, buffer.Length);
    }
    return Content("chunk uploaded", "text/plain");
}

This method will be called multiple times for each chunk and for each file being uploaded. It will pass as parameter the chunk size and the filename. I am not sure as to whether you could use a HttpPostedFileBase as action parameter because the name is not deterministic.

Look here:

$("#uploader").pluploadQueue({
         // General settings
         runtimes:  silverlight ,
         url:  /Home/Upload ,
         max_file_size:  10mb ,
         chunk_size:  1mb ,
         unique_names: true,
         multiple_queues: false,

         // Resize images on clientside if we can
         resize: { width: 320, height: 240, quality: 90 },

         // Specify what files to browse for
         filters: [
            { title: "Image files", extensions: "jpg,gif,png" },
            { title: "Zip files", extensions: "zip" }
        ],

         // Silverlight settings
         silverlight_xap_url:  ../../../Scripts/upload/plupload.silverlight.xap 
      });

      // Client side form validation
      $( form ).submit(function (e) {
         var uploader = $( #uploader ).pluploadQueue();

         // Files in queue upload them first
         if (uploader.files.length > 0) {
            // When all files are uploaded submit form
            uploader.bind( StateChanged , function () {
               if (uploader.files.length === (uploader.total.uploaded + uploader.total.failed)) {
                  $( form )[0].submit();
               }
            });

            uploader.start();
         } else {
            alert( You must queue at least one file. );
         }

         return false;
      });

And in Controller:

[HttpPost]
public string Upload(  ) {
          HttpPostedFileBase FileData = Request.Files[0];

          if ( FileData.ContentLength > 0 ) {
             var fileName = Path.GetFileName( FileData.FileName );
             var path = Path.Combine( Server.MapPath( "~/Content" ), fileName );
             FileData.SaveAs( path );
          }

          return "Files was uploaded successfully!";
       }

That s all...No chunk is needed in Controller...