Inder,
Do you have a real physical PLC connected to your Personal Computer (PC)? If you do not, then you cannot "Write to PLC", even in the Simulator mode. In most Simulator programs, it is not necessary to do the "write" step, because the PLC may not exist or not be connected. Instead there is a menu to run in simulation mode. Click on the Simulator ON icon, then click on the icon to switch the PLC to RUN mode.