The manual can be downloaded here:
http://bama.edebris.com/manuals/rme/rme4350a/In answer to the question: From the bottom right hand corner of the escutcheon, about an inch above and an inch to the left, there is a toggle switch with a bat handle that moves horizontally. Another way to find it is immediately below the round S-Meter on the right side.
The calibrator is ON when the switch handle is flipped to the right, OFF when it points to the left.