HomeController.cs

[HttpPost]
public string Merge(DocumentViewModel model)
{
    // convert the template to a byte array
    byte[] document = 
        Convert.FromBase64String(model.BinaryDocument);

    // load the the data source
    DataSet ds = new DataSet();
    ds.ReadXml(Server.MapPath("/App_Data/data/sample_db.xml"),
        XmlReadMode.Auto);

    byte[] sDocument;

    // create a new ServerTextControl and MailMerge component
    using (TXTextControl.ServerTextControl tx =
        new TXTextControl.ServerTextControl())
    {
        MailMerge mm = new MailMerge();
        mm.TextComponent = tx;

        // load the template and merge
        mm.LoadTemplateFromMemory(document, 
            FileFormat.InternalUnicodeFormat);
        mm.Merge(ds.Tables[0], true);

        // save the document
        mm.SaveDocumentToMemory(out sDocument, 
            TXTextControl.BinaryStreamType.InternalUnicodeFormat, null);
    }

    // encode and return the merged document
    return Convert.ToBase64String(sDocument);
}