To find the flat from the Airport is easy. Take the Metro m3 direct to Kerameikos and Exit Konstantinopolus (ca.40minutes, 18€ for two persons). To reach the apartment walk left along the railway 300-400 mtr. A market for daily belongings is nearby across the railway (300 mtr.) Ring at the 5th floor where the owner lives.
It is a very nice flat but the only problem is the loud railway across street. But if you are out all day, walking through Athens anyway, it might not bather you to much.
Thank you very much we had a nice stay at your place.