Yeah, the fields can be walked on saturday, from about noon onwards.
Since I haven't got everybody's email address, I'm posting the route to the field up here:
From the A10 Ring Amsterdam, take exit S102, and follow the signs towards IJmuiden. After a while you will see an exit on the lefthand side called Spaarnwoude. Take this exit and go left again immediately. You are now driving along a small industrial area, with a number of junkyards. Just follow this road, and you will be led around a right curve. After about a quarter of a mile, you will see a green fence on the right side of the road. Drive through the fence and proceed down the track along the canal. At the end of the track is the safe zone, and you can park along the track.
A little map to the site:
PS It's turned on it's side. The left hand side of the map is north.