To add intro content that will appear on all Category pages, click the main ECOMMERCE link, then the INTRO button at the top-right.
To add intro content that will only appear at the top of the specific Category - navigate to the relevant Category and click the INTRO button found there.
When editing a Category - you can also click the top INTRO button, or the Category Header edit button.
As the main category does not have its own settings area, you need to go to the main ECOMMERCE page to access it.