Franck,
The CallersPath in the FPOptohandler.vi should be wired to the constant CurrentVI's Path found in the File I/O --> File Constants Palatte. However, you should not be needing to do anything with the FPOptoHandler.vi since it is a subVI of the FieldPoint Optomux VI's. Is there some function you are trying to implement that is not covered by the FieldPoint Optomux VI's?
Regards,
Aaron
LabVIEW Champion, CLA, CPI