One way to get to Fernando's....if you are standing in front of La Merced, facing North on 1a Ca, The street to your west is 6a Norte.  Walk one short block north on 6a Norte to Calle Camposeco.  Then go west to the end of the block.  You'll probably be able to smell Fernando's latest batch of beans roasting, from down the block. Fernando's is at the corner on the left on 7th Avenida to the north a bit right, next door to the Posada de la Merced.