I have no ciao system in reach of my hands, so I can't test it right now, but I'm pretty sure that in ciao you can't use the operators defined in a different file. I have encountered it while doing some work with ciao and the solution I used was to simply declare all operators (which was actually only one) in every module. I'm actually quite curious if there is any smarter way to do it.
Quoting Jeff Donner <jdonner(a)cs.nmsu.edu>:
?- Who was the secretary of the department. {SYNTAX ERROR: (lns 6-6) operator expected after expression
^^^^^^^^^^^^^^^^^^ you get the error because ciao doesn't see the operators declared in a file, although any predicate declared in that file would see those operators.
Who ** here ** was the secretary of the department . }
no ?-