Hi Rend,
The codes written for co-simulation have been made with regard to the format of the vectors generated in OptiSystem. For example, for an electrical signal OptiSystem creates a time vector and a magnitude vector separately. You must know how to refer to each of these vectors in your code to be able to access them and process their data by Matlab. A good first step to understand these is to start with the videos created by Optiwave and tutorials available in OptiSystem samples in Matlab co-simulation section.
Regards