Tlatelolco is a district of Mexico City, the site of the Aztecs' last resistance against Spanish rule and the site of a massacre of student protestors by police and military on 2 October 1968.
Take the Green line train to Tlatelolco. There are two exits out of the station, take the one that opens to a Basketball court, not the open that opens to a street. Continue past the basketball court through a park until you reach a street. Turn left and continue until you get to a 30ish story white building. You have arrived.