The standard lexicon and grammar files are available here:
You can switch between them using the "Lexicon" menu, or by pressing the "Load Grammar/Lexicon" button and editing the URL suitably.
If you want to modify them to create your own definitions: choose one to start from, download all three files in that directory, and save them somewhere local (in their own dedicated directory). You can then edit them to your heart's content; and load them into the parser using the "Load Grammar/Lexicon" button.