The Oktoberfest takes place on the Theresienwiese (the big field on the lower left) near Munich City Centre. It is perfectly walkable from the Central Railway station or ‘Hauptbahnhof’ and the closest U-Bahn station to it Oktoberfest entrance is Theresienwiese. Be aware the U-Bahn gets very very busy during prime times.
Finding it on foot is easy as you’ll be able to follow the masses of people walking there. As you look at the map, the tents are located in the upper left part of the field. The fun-fair more or less on the lower left and right. Look at the satellite view which actually shows the tents.