from the airport: you can take a taxi or remis (cost USD 50 aprox), or a bus during the day, to downtown or to 'Tres Cruces' bus terminal.
from 'Tres Cruces' bus terminal: you can take a taxi (cost USD 5 aprox), or a bus with numbers CA1, 180, 181, 187, 188 (cost USD 0,5 aprox) and get down at 18 de Julio in front of the city hall (Intendencia), the hostel is 5 blocks south east from there.