Argentina's new government has not yet approved a law to allow people to bring goods into the country without paying a tax. Until then, travelers must fill out an affidavit detailing what they bought abroad.
The procedure is carried out at the customs posts in the terminals and airports. It is important to arrive with enough time to do this, otherwise, Customs could demand that taxes be paid on those products, as if they had been acquired during the trip. The AFIP has a step-by-step guide for this procedure (accessed here).