Hello Khadija Omran,
Actually, we calculate Minimum BER in optisystem using BER Analyzer which is found in electrical part of visualizer component library.
It is easy to find to Min. BER and Q-factor.
Here i upload an osd file
When you will double click on BER analyser, you will get value of Min. BER, Q-factor. You can get structure of eye diagram by just get tick on Show eye diagram tab in BER analyser window.