Park at the bottom of the lot, as close to the Plaza entrance as you can. Walk past the Reston Community Center (on your left) toward the benches at the lake.
The Church is to the immediate right of the benches and left of the coffee shop.
You'll see the marquis for the Church. Go up the stairs to enter the Church sanctuary or use the sidewalk located to the right of the Church, between the coffee shop & the Church, to enter the plaza room.