Take the Beltway to the last Maryland exit (“Carderock/Great Falls”), go about one mile north (away from DC) and exit at Carderock (the first exit). Turn left after the Stop sign, and follow the twisty road into the park and through the underpass under the canal. At the T, turn right and then immediately turn left into the first parking lot (shown by the blue mark on the map).  If you are coming the first time, be aware there is sometimes traffic on the Beltway (you may want to give yourself a few extra minutes). If the Beltway is really backed up on a given day, there are some alternate routes you can take from either the end of Seven Locks Road or Potomac Village (see a map to scope them out).

Map showing Carderock parking lot where to meet: http://goo.gl/maps/3uM6T