From the Garden State Parkway:

Exit 135 for Clark & Westfield. 
Take Central Ave thru Clark and Westfield to END.
Make a RIGHT onto E Broad St and a quick LEFT onto Mountain Ave.
At traffic light, make a LEFT onto New Providence Road.
Cross Route 22.
Make 1st RIGHT onto Birch Hill and 1st RIGHT onto Constitution Plaza.

From points west:

Take Route 22 East.  Go through Watchung and Scotch Plains. 
Enter Mountainside.
Exit at New Providence Rd.  At end of ramp, make LEFT.
Cross Route 22
Make 1st RIGHT onto Birch Hill and 1st RIGHT onto Constitution Plaza.

From points east:

Take Route 22 West.  Go through Union and Springfield. 
Enter Mountainside.
At first traffic light, exit onto New Providence Rd.  Make RIGHT at end of ramp.
Make 1st RIGHT onto Birch Hill and 1st RIGHT onto Constitution Plaza