Dear Jonathan,
Please refer to the example (Mach Zehnder Interferometer.osd) at the following location for guidance. The theory of the Mach Zehnder Interferometer component is described in the datasheet that can be accessed from the Help button in the component properties popup window.
C:\Users\USER NAME\Documents\OptiSystem 18.0 Samples\Component sample files\Filters Library\Optical
Regards,
Ahmad