Most credit cards are widely accepted in Brasília. It is possible to withdraw cash from an ATM if you have an international credit card. Maintain some cash for minor expenses such as taxi, stores, coffee shops etc. US Dollars and Euro can be exchanged in exchange offices and banks. Keep your money and personal belongings and documents (e.g. passport) with you or in a safety box while in the hotel, airport etc.
See Real exchange rates in: http://www4.bcb.gov.br/pec/taxas/ingl/ptaxnpesq.asp?id=quotations