Ok.. I looked it up for you. If you come in from the port (Pireaus) on the green Metro line you need to transfer to the blue line and then to the red line to get to the Acropoli station This will dump you right at the main entrance to the Acropolis site. The point where you transfered from the blue to the red line is also the Syntagma station (Constitution Square) The Plaka area basically sits between these two stations so it is walkable form the Acropolis station. But the Syntagma is closer and if you get out there you can watch the changing of the guard at the tomb... But if you do we we did, you can see the acropolis and the other temples at the site and end up on the back entrance near the Thissio station (ask someone where the Metro station is or look for the M sign) then you can take the train over to the Syntagma stop to get to the plaka area.
DON'T get off at the Monastiraki stop. Yes its very close to the Acropolis but is is on the opposite side from the entrance and you will have to walk all the way around to get in (voice of experience here) We wasted a good 45 minutes doing this.
Carnival Cruise Lines:
Fantasy 90, Celebration 95, Victory 05, Conquest 05, Spirit 6/06, Freedom 6/08, Holiday 7/08, Victory 6/09, Valor 9/09, Dream 6/10, Splendor 8/10,Conquest 8/11,Magic 11/11,Triumph 4/12,Spirit 7/12,Freedom 11/12, Triumph 2/13 - The fire cruise
NCL Dream October 2006, RCCL Mariner OTS June 2007