DO you still have some sort of proof of purchase ? ( ie. the e-mail you were sent at time of purchase - credit card statement with purchase -etc. ) If so you may be able to contact Bethesda support and if they can verify the purchase they might be able to help.
Though currently the download is no longer available since the website that was selling the DLC has been shut down and the only method of obtaining it currently is purchasing the entire GOTY Deluxe edition from STEAM.