A last minute change in the parking situation (DCR refused to give us a permit for the Houghton's pond area) means that there is a new set of directions to the event parking.
NEOC
schedule shows this change.
Trailside Museum Gmap
link
From I-93 (Route 128 in Canton) take Exit 2A onto Route 138 N. Watch on the right for Orienteering signs for the parking area near the Trailside Museum.