EMBASSY OF THE UNITED STATES BUENOS AIRES, ARGENTINA

Once you have received the "Instruction Package for Immigrant Visa Applicants", the visa applicant must follow the instructions regarding the visa fees and documentation.

If you filed an Immigrant visa petition with USCIS and it was sent to the NVC:

Do not send any payments to the Portsmouth, NH address, as they will be returned to you. Send the payment and fee bill(s) to the address provided by the National Visa Center.
When they have received your immigrant visa fee they will send you the immigrant visa application (Form DS-230, part I) along with further instructions regarding all the required documents for you to gather and send back to the NVC.  Once the NVC has reviewed and determined that your case is documentarily qualified, they will schedule the visa interview appointment which will take place at the US Embassy in Buenos Aires.

If you filed an I-129F petition (for K visas) with USCIS and it was sent to the NVC:

The NVC will assign a case number starting with BNS and they will send it to the Immigrant Visa Unit at the US Embassy in Buenos Aires.  Once this office has received your K visa case from the National Visa Center, the Immigrant Visa Unit (IV Unit) will send you an instruction packet (PDF-1691KB) including a checklist of required documents.  Once you have completed gathering and completing the information and forms required and we have received the corresponding copies from you, the IV Unit will schedule your final visa interview.