I think the easiest way to find this place is to get to the Bow Bridge (just google: Central Park Bow Bridge) and then walk up using any dark footpath towards Belvedere Castle. It is very safe but remember to leave the place before 1 AM because police closes park at that time.
The area around the Boathouse is called The Ramble. Enter the park from the east side at the entrance at 72nd Street. AYOR. Police in plane clothes.