From Southern New Jersey and points south:

Take I-287 NORTH to Morristown EXIT 35 (C-124 Madison Ave.). 
Turn left at the end of the ramp and continue on C-124 WEST (South Street). Follow directions from Morristown Green at bottom.

From New England and points north: 

Take I-287 SOUTH to Morristown EXIT 35 (C-124 Madison Ave.).  Bear right at the end of the ramp. Turn right onto C-124 WEST (South Street). Follow directions from Morristown Green at bottom.

From northern Pennsylvania and points west: 

Take I-80 EAST 
Next take I-287 SOUTH, to Morristown EXIT 35 (C-124 Madison. Ave.). Bear right at the end of the ramp. Turn right onto C-124 WEST (South Street). Follow directions from Morristown Green at bottom.

From New York City via the George Washington Bridge and Interstate 80 West: 

I-95 WEST to I-80 WEST. 
Next take I-287 SOUTH, to Morristown EXIT 35 (C-124 Madison Ave.). Bear right at the end of the ramp. Turn right onto C-124 WEST (South Street). Follow directions from Morristown Green at bottom.

From New York City via the Lincoln Tunnel:

From Morristown Green onto the Saint Mary's Abbey/Delbarton School Campus:

Proceed straight one mile to the Morristown Green.
Travel three sides of the Green (do NOT take S-202).
Follow sign for C-510 WEST (Washington Street).
Washington Street eventually becomes Mendham Road (Route 24 westbound).
Follow Mendham Road approx. 3 miles until you come to the East Gate entrance of the Saint Mary's Abbey and Delbarton School campus.  Turn left onto the East Gate Road.
Follow the East Gate Road and the large green road signs along the way that will lead you to the Retreat Center, about 1/2 a mile from Mendham Road (see campus map below).  Park right outside the Center entrance or in any designed Visitor lot.