Hi hadi,
1- Bit rate in PRBS is usually set as bit rate in the global parameters by using script mode. However, the value you choose for the one in PRBS is the one used in the simulation as Bit rate.
2- In order for the cyclic prefix effect to be seen, you need to use BER Test Set instead of BER Analyzer. I have explained that explicitly in this very page.
3- You can download Optisystem 13.0.3 64 bit from “Evaluation License Download” section in your profile.
Cheers!