Jimbo is quite right ... the truss-rod adjustment nut can be found directly behind the main cross strut.
The bi-flex truss-rod is housed in a steel channel that bends thus moving the neck ... some truss-rod setups have the bare rod against the neck and fingerboard wood so high compression could potentually damage the wood, that can't happen with our truss-rods.
Strings do not need to be removed to adjust the truss-rod ... I'm surprised your guitar "fettler" didn't know this
... please make sure he knows what he's doing before letting him make any major adjustments.