Take the escalators going to the basement, where the trains depart from. You should see the (small) JR ticket office right in front of you, with the entrances to JR and Keisei platforms on the left.

Turn around, and there should be a 'Travel Service Center' where you can exchange your voucher for a JR Rail Pass.

[ed. escalators, not elevators ]

[This message has been edited by monahos (edited 02-12-2003).]