Peter.nc,
The best way to find it is:
Take a taxi or tram to Dam Square.
Stand facing the entrance to the Grand Hotel Krasnapolsky. There is a street that runs along the right hand side of the hotel (I forgot the name of the street). The Old Man shop is on the right hand side of this street, just down from the hotel. The knives are, or I should say were, in the back portion of the shop. The front of the shop is full of... well, you'll see.
If you come to the canal you have gone too far.
Have a good time in Amsterdam. I worked there for a while and visited at least 5 times a year after that. A great place. Try the Indonesian food. It's some of the best in the world.
Blackdog