If you come by subway the easiest stop is "Les Halles" station on the line 4. Once on the platform take exit 3 "Rambuteau".
The station "Châtelet" on line 1,7,11,14 will also work. In this case take the quickest exit and once outside find your way to "3 Rue de la Cossonnerie" it should be less than 300 meters away.