From Roy Pascarella:

The handle portion of the safe sticks out to the left.  Situate the arm
on the handle not the round portion of the handle.  Use your down arrow
key to retract the arm as far as it will go.  Then click on the switch
on the upper panel that extends the arm...  I'll bet it'll open
immediately.  The problem was one of perspective as I was (and I think
you've been) extending the arm until it was over the handle and then
clicking the switch which would cause it to overextend.