If you mean, where you can find plugins to download them, there is a section on the forums named "Plugins and Third-party tools". In this section, users post plugins they make for MCreator. Some users also give their plugins on some Discord servers like the MoreCube Discord server. However, if you mean where is the folder with plugins, go in Users/yourUser/.mcreator/plugins for external plugins, and the folder where you have installed MCreator/plugins for MCreator plugins. You can also see all plugins Mcreator has loaded into your Preferences (Manage plugins).
If you mean, where you can find plugins to download them, there is a section on the forums named "Plugins and Third-party tools". In this section, users post plugins they make for MCreator. Some users also give their plugins on some Discord servers like the MoreCube Discord server. However, if you mean where is the folder with plugins, go in Users/yourUser/.mcreator/plugins for external plugins, and the folder where you have installed MCreator/plugins for MCreator plugins. You can also see all plugins Mcreator has loaded into your Preferences (Manage plugins).
Where do you find the folder on mac?