1) Same as from Ezeiza Airport (EZE) or Aeroparque Jorge Newbery (AEP), you can book in advance a private taxi from the company GreetersBA, you have to book it online through the following website: http://www.greetersba.com/booking.php
When you arrive, the driver will be waiting for you in the ARRIVALS HALL holding a sign with your name + Milhouse hostel logo. The price of the transfer is around U$D 25 (per car, not per person, up to 3 people) and you pay directly to the driver in cash, the trip takes about 40 minutes.
2) You can also take a regular transfer called Manuel Tienda León, the price is AR$ 220 and it takes 1 hour. The transfer will drop you off in Puerto Madero station (Avenida Eduardo Madero 401), from there you have a Subway station call Alem (line B) one block away in Corrientes Avenue, you get off in Uruguay Station and walk 5 blocks up to Avenida de Mayo.
3) Finally, the cheapest option is by public transport, just in front of the airport you have San Martín Train station, you have to go to the last stop that is called Retiro, from there you can take the subway C direction Constitución, until Avenida de Mayo Station, from there walk 2 blocks. Takes around 1 hour.