From the city centre (e.g. train station) take bus 3 to the south of town until the stop Mariazeller Straße/Info Point. You will see the ramps of the highway A1 (another name of this highway is Westautobahn). Follow the ramp a few meters up, then there's a little place to your right hand where cars can stop.
Take the bus number 1541 from the train station (back side, lane X) to Völlerndorf. Head a little back and then up the hill to come to the service station St Pölten on the highway A1. This service station has an underpass under the highway, i.e. it is is for both directions.