If you take public transportation, you should get off at Syntagma square, find McDonalds (Ermou 2) and then walk downhill until you find Ermou 44 on your right (there is a Mother Care store at the ground floor of the building).

When you arrive, ring the bell of Athens Direct Travel and then take the elevator to the 5th floor, while the rest of the group can go to the 3rd floor, where the apt is located (apt number 9). Either me, or someone from the staff of Athens Direct Travel will welcome you, give everyone the tour of the apartment and provide the keys!