Rockburn Branch Park can be accessed through two main automobile entrances with parking, or by any of several trails from surrounding roads and neighborhoods. During weekends and many weeknights, the fields are very busy with tons of youth sports. Keep looking around - I've never had to turn away because of parking.
The cache is hidden in the northern section of the park. You should be able to get within about 90 feet of the cache without leaving the trail. Just down the hill from the cache is the Rockburn Branch, the park's namesake.