Walk on the Leidsestraat due east from the Leidseplein until you hit the Singel, and it will be right in front of you, NE corner (The square is called Koningsplein).
You can actually get a nice view of the store and the street on the excellent Koningsplein webcam (and don't forget to turn up the volume - hearing the approaching tram bells is a nice touch):
http://www.terena.nl/~dick/cam2.asx (due to building renovations, some scaffolding is obstructing the view)
Mac