If you're outside of the U.S. and you plan to vote then you should visit www.fvap.gov the Federal Voting Assistance Program. This is the official U.S. government site for voters who are overseas.
On this site you will be able to register to vote and to request an absentee ballot. There is a step by step diagram of what you will need to do to vote through their site. In addition, there are many resources and answers to frequently asked questions to help you through the overseas voting process.
The site is broken down into two sections - One for for active-duty members of the Armed Forces, Merchant Marine, Public Health Service, NOAA, and their family members and the other is for United States citizens who are living outside the U.S. for work, school or other reasons.
If you have specific questions about absentee voting then contact the Voting Assistance Officer at the U.S. Embassy in Buenos Aires at 011-5777-4818 or email firstname.lastname@example.org .
I encourage you to visit the site and register to vote. It's going to be an interesting election and just because you're out of the country doesn't mean you can't participate.