You need an appointment to visit USCIS Athens.  You can make an appointment by phone at: 210-720-2404 and 210-720-2405 or at USCIS Athens office will forward you a confirmation email within 48 hours.

Address: U.S. Citizenship and Immigration Services in Athens
U.S. Embassy in Athens
91 Vasilisis Sophias Ave
10160 Athens, Greece