Dear Vamsi,
you can modify the examples in OptiSystem Example library on the desired modulation scheme and sweep the length of fiber/channel then in the report page you could display BER versus Length, or OSNR versus length. You need to use the project browser and access these parameters from the related components once you expand each category of that component and drag the specific parameter (BER, length, OSNR, etc…) and place it on the desired axes on the graph that you created in the project browser.
Regards,
Ahmad