Ataco, or more formerly known as Concepción de Ataco, is very easy to come and go from. Buses pass by the edge of town in front of the Parque Ecologico San Francisco – although you wont find this park on google maps (see photos).
From San Salvador
From Juayuá take 249 heading to Ahuachapán. This bus passes Ataco after about 25 minutes.
From La Libertad (El Tunco, Playa Sunzal, El Zonte, La Mizata)
To get here from the coast take the direct bus from La Libertad at either 6am, ~1:45pm to Sonsonate. This bus will pass El Tunco, Playa Sunzal, El Zonte and La Mizata and takes about 2hrs. Then the 249 to Ahuachapán which will pass Ataco along the way.
From Santa Ana
From Santa Ana take the 238 bus to Juayuá. Walk two blocks from where you are dropped off to the bus stop next to the Scotia Bank and wait at the corner. Take the 249 bus heading to Ahuachapán and get off in Ataco. Alternately, go to Ahuachapán first and then switch to the bus heading to Sonsonate which will pass Ataco on the way. Trip time is about 1hr 45min in total. Both buses leave the Terminal Francisco Lara Pineda.