To attend the PASI conference you may require a tourist visa to Brazil and to Argentina.
U.S. citizens do not require tourist visa.
Other nationalities exempt from a visa can be found in:
Contacts in US: Consular Section of the Argentine Embassy, 1811 Q St., NW, Washington, DC 20009 (202/238-6460) or the nearest Consulate: CA (213/954-9155), FL (305/373-1889), GA (404/880-0805, IL (312/819-2610), NY (212/603-0400) or TX (713/871-8935). Internet: www.embassyofargentina-usa.org
A tourist visa is required from holders of US passports and several other countries to enter Brazil.
Nationalities exempt from a visa can be found in: http://www.visaworld.co.uk/_brazil.html
Information for applying to a visa for traveling to Brazil can be found in http://www.brasilemb.org/consulado/consular_visa_tourism.shtml
Contacts in US: Brazilian Embassy (Consular Section), 3009 Whitehaven St. NW, Washington, DC 20008 (202/238-2828) or nearest Consulate: CA (323-651-2664 or 415-981-8170), FL (305-285-6200), IL (312-464-0244), MA (617-542-4000), NY (917-777-7777) or TX (713-961-3063). Internet: www.brasilemb.org
You should allow at least one month to obtain your visa, whether for Argentina or Brazil.
Two agencies that provide visa service in the U.S. are the following:
Visa Express: http://www.visa4you.net/
Passports and Visas: http://www.passportsandvisas.com/