To get to it, go upstairs from the control room and to the top of the metal stairs outside. B636 is the furthest building to the left. To get there, enter the grey door opposite the restrooms upstairs from the control room. Forward and to your right are a set of white racks, one of which is labelled B620B-38. The DIRC water system is located on the hall floor to the east of the EH, where there's a checklist specific to this system in a blue binder.
Instructions are on a laminated page on top of the chiller unit.