Hello,
PLECS comes in two versions- Standalone and Blockset for use with MATLAB/Simulink. The latter version requires a license from us (separate from MATLAB) for full operation including changing and saving models, but you are able to use the free PLECS Viewer license if you have a developed PLECS model available you want to open. If you just have thermal descriptions (.xml files) from a semiconductor vendor, you will need to request a trial license at www.plexim.com/trial or purchase a full license to build these into a circuit model. I suspect this previous sentence is what is really relevant in your case.
Regards,
Kris
