I'm assuming you're looking for the Willowwood Arboretum that is part of the Morris County Park System. According to their web site, it is located in Chester; check the Directions tab for map and written directions.

If you're ever in eastern Monmouth County, check out Deep Cut Gardens on Red Hill Road in Middletown, 1.7 miles from Exit 114 of the Garden State Parkway. www.monmouthcountyparks.com for more info. 54 acres of gardens (both formal and informal), lawns, woods, meadows, a display greenhouse and a horticultural library containing over 3000 volumes.

Here is a link that might be useful: Willowwood Arboretum