By Metro:

From the airport you could take the metro direct line number 3  until Monastiraki Square. The train runs every 30 min.The journey is about 40 minutes. It operates from 06:35- 23:35. A single fare costs 6 euro(2010). Don\'t forget to validate your ticket and take care of your belongings .
When you get out of  the station, (exit Athinas street) you should turn left  and walk on Athinas street for 300 meters. You should turn left again on the street after the first traffic light. This is ARISTOGITONOS street, the pedestrian street were is the fruit market.  You will see Athens Center Square hotel few meters on your left.

By Bus:

From the airport you could take the bus number X95 , cost 3,20 euro till station Syntagma Square.  From there walk on Ermou Street till you find Athinas street.Walk for 300 meters and after the first traffic light you will be on ARISTOGITONOS street . It is a pedestrian street where is the fruit market.  You will see Athens Center Square hotel few meters on your left. This journey may be a bit longer than the metro 
 
 

By Taxi:
The journey is approximately 35 - 45 minutes. The cost is not more than 45 euro, with an extra charge during the evening.