In order to get a DNI you first must apply for temporary residency.
The temorary residency your Social Security income will qualify you for is known as the Visa Pensionada.
You can get the benefits letter in Spanish from the Special Benefits office of Social Security at US Embassy in Buenos Aires.
It will not require translation but must be "legalized" in Argentina. The office that does the legalization is in Retiro (on Aranales near the Plaza San Martin).
I believe you will need an advance appoitment to get the letter at the US Embassy as well as to get the letter legalized in Retiro
I don´t know the exact name or address of the latter, but I´m sure someone else will provide it..