Archbishop Nikola Eterovic
General Secretariat: Palazzo Del Bramante,
Via Della Conciliazione, 34, 00193
Rome, Italy
Telephone: 69.88.48.21 or 69.88.43.24
Fax: 69.88.33.92
That is the basic way I suppose. Depending on the issue you could go through contact with the USCCB (United States Conference of Catholic Bishops) or some other method.
But that address and the phone numbers and fax should be current.
I am not sure what you need it for but a direct letter might get lost in the shuffle.
I can try to find a step by step way to move up the chain if you want. I make no promises but I am good at finding stuff.
But if you want direct contact the address is above. The address (Via Della Conciliazione, 34, 00193) seems to also be connected to the Congregation for Oriental Churches. So it is likely a personal route to a residence or old office and not directly to one of the Synod Offices. He is an Oriental Rite Bishop and Titular Archbishop of Siscia so that makes sense.
Also I think it is still the case that the American in Rome working for the General Secretary of the Synod of Bishops would be Msgr. John Abruzzese and he can likely be contacted through:
Residence for Americans Working in the Roman Curia:
Villa Stritch, Via della Nocetta 63, 00164 Rome, Italy.
Tel: (011.39.06) 66.16.2662; 66.16.2676; Fax: 66.16.2700.