Have you reviewed the process in our User Manual? Here is the description below:
Creating a New Library in PLECS Blockset
To create a new component library, open the PLECS Extras library (in the Simulink library navigator) and copy the PLECS Library block into a Simulink model or library. The Simulink model must be named (i.e. saved) before you can copy components from the component library.
To add the new library to the library browser it has to be added to the list of user libraries in the PLECS Preferences window, which is accessed from the File menu when inside the PLECS Circuit block.
You will also need to create a MATLAB path link to the folder containing this library model file.
Please let us know if you are successful or have specific questions.