Did you find the answer? If not here it is:
Lost or damaged the red prepaid return mailer?: just place up to two DVDs in another Netflix red prepaid return mailer. If you are on a 1-out-at-a-time plan, or can't locate any red return mailers, please send the DVD back to us inside its white sleeve, in your own return envelope with sufficient postage to the address listed below. The white sleeve includes a special barcode that we can use to associate this returned disc with your Netflix account.
Netflix
P.O. Box 49021
San Jose, California 95161-9959
