Please verify you have an install of Java. If you do and it continues to not work correctly, please update to the latest version of Java. If you still have trouble please post an issue here.
How to Use:
Run the executable jar file or the executable. When the window shows, type in your mod id into the text field next to the label "Mod ID".
If you wish to use a custom directory, click "..." to select it.
If you want to create an item, type in your item's name and texture name into the two middle text fields. If your item is a tool, click the checkbox that says "Is Tool?". Once you have finished, click the button that says "Generate Item JSON".
If you want to create a block, type in your block's name and texture name into the last two text fields. Once you have finished, click the button that says "Generate Block JSON".
If you used a custom directory, your files will be inside the directory. If not, your newly generated JSONs will be placed into a folder in the same directory as the jar called "export". Your files will be inside the corresponding folders (Blockstates and Models)