Take the train to Amersfoort (not Amersfoort Schothorst). From there you can walk to the Amershof in about 10 minutes. When you leave the station (in the direction of the center of Amersfoort) walk to the road right in front of it and turn left. You follow this road and turn into the second road to the right. This is the Snouckaertlaan. A little furtheron you turn left into the Burgemeester de Widtstraat. A little downhill you can turn right into the parkinglot of the Amershof. You're there.
Get onto the A28 from Utrecht to knooppunt Hoevelaken. Take exit 8 Amersfoort and turn left at the traffic lights. You'll have to follow this road for quiet a while. Mind your speed, there are a lot of camera's down this road. On the crossing with the KFC to your right, turn left. This is the Utrechtseweg. Now you take the first road to the right, this is the Snouckaertlaan. Pass the Amershof and turn right once more into the Burgemeester de Widtstraat. A little downhill you can turn right into a parkinglot. You're there.
For more info about the route to take you can look at Routenet.