I just received a book from the USA. I entered the page of the AFIP, and with some help I added " MIS APLICACIONES WEB". This was the hard part. Then you start the form 4550, NUEVO, plug in the Correo Argentino card number, follow thru, and print it.
Take the card and the form and go to Retiro, pay as indicated (books are free of custom duties), pls also note that there is a 25 USD free pass per person per year. There is a 40 pesos fee any way.
PS: you need a CLAVE FISCAL, of course.