The Editor is opened by right-clicking a module in your VASSAL module library view and choosing “Edit Module”.
I don’t mean to be unkind, but this import/export functionality is only going to be of use if you are heavy into module editing, and it doesn’t sound like you are since you didn’t even recognize that as the Editor.
You may not need any of this for your VASSAL use case–e.g., if you only play modules.