DO NOT GO TO RETIRO!
Instead, walk 3 blocks over to Av. Cordoba. Get on the Subte D towards "Congreso de Tucaman". (The other direction is "Catedral", and you don't want that.)
Remember, the Subte's "run on the
left here because the British built the trains, trams, & first few subway lines. So you'll have to cross Cordoba before going down underground.
Then just take the Subte to Juramento. Get off and walk down hill on Juramento about 5 blocks. You pass a little park and then a bigger park right as you get to the train tracks. Cross them and turn left.
Taking the train from Retiro will save you a peso or two on that ticket, but you'll have to take a bus down to Retiro, so you lose a bit there. And Retiro is fairly scummy, so there's that as well.
The walk down Juramento beats the walk around Retiro 7 days a week. Plus, you get to see a different neighborhood (Belgrano).
http://es.wikipedia.org/wiki/L%C3%ADnea_D_(Subte_de_Buenos_Aires)