Hi Pavi Karthikeyan,
If you are using MATLAB 8.2 (R2013b), you can find 'powergui' block at Simscape->SimPowerSystems->Second Generation. You can also type 'powergui' in the search tab provided in the Simulink Library.
Note that SImPowerSystems Toolbox is not available by default in the Student Suite as per this link. You may have to go for the Physical Modeling and Simulation suite of products, as suggested by the same link, in Student version to get SimPowerSystem Toolbox.
To check if you have the SimPowerSystem Toolbox installed and available for use in your MATLAB type the following on your MATLAB Command Prompt:
If the above returns '1', then you have it installed.
Hope this helps,