Raja, Are you using PLECS Standalone or PLECS Blockset?
Assuming you are using PLECS Standalone, you can initialize the parameters from an m-file or you can use MATLAB to run PLECS Standalone (note the associated tutorial in the links below the video).
If you are using PLECS Blockset, then you can directly assign the variables in the MATLAB workspace by entering them into the console, running an m-file, or assigning the variables in the InitFcn (Simulink menu bar of your model: File > Model Properties > Callbacks → InitFcn.)