From the TNP site:

I-95 to Exit 58
Go south on Rte 77 to Guilford Green.
Turn right onto Broad Street, go one half block and then left onto Whitfield Street (remaining on Rte 77).
Go past the Green and turn right onto Rte 146 (Boston Street)
Follow Route 146 for 2.4 mi (After the Guilford Green, Boston Street becomes Water Street).
After passing under the railroad, continue straight on to Sachems Head Road for 0.6 mi.
Bear right on Colonial Road and continue 0.2 mi.
Turn right on Old Sachems Head Road and continue 0.3 mi.
Turn right on Trolley Road. Boat launch is at the end of the road.
Parking is very limited. Carpool if possible.