From I-89, take Exit #14 (Route 2 east). Continue on Route 2 east for approx. 2 miles to Aviation Ave. (look for blue sign marked "U.S. Customs and Border Protection - Air Cargo and Intl. Arrivals"). Turn left onto Aviation Ave. and continue to end (U.S. Customs and Border Protection office is on left).
After hours clearance should be arranged in advance during office hours.