If you are travelling by train, the store is really easy to find. Take on the railwaystation of Amsterdam Zuid not the main exit, but the backentrance at the far end of the station. Go down the stairs, and turn right. Walk under the highway and subwaybridges, followed in a short distance by a tunnel under another local street. Keep at the right side of the street. If you see a office build over the road, you are already too far. But a sharp eye has not missed the trains in the shopwindow, I think .

If not, take exit "Noord" (North) of the station. Cross the square between the offices and walk under the busstation. Cross the cyclepath and the little grassfield. The road in front of you is the Strawinskilaan. Turn left untill the Parnassusweg, and go right. The Märklinstore will appear at your right hand.

And some pictures of the new store. In the first picture you can see in the background the office builded over the road. At that point, you are too far.