How to add a categorie to top menu

[Ellen4strings]

We have a functioning prestashop but now I wish to add a category to the top menu. This categorie then shows different subcategories where you can chose a product.This is the website: www.4strings.be

My questions:
1. I've added the following category to the homepage and in the red top menu: "Huur Instrument". The problem is that I cannot keep it there when I go to different pages. e.g.: click on "Vioolsnaren" and this category disappears in the red top menu. I've looked at the top menu module but cannot fix it this way.

2. Just like the other categories e.g. Viiolsnaren, this new category of "Huur Instrument" also needs subpages. How do I go about making them. Where do I make the subpages of this category and how do I get them to show up on the homepage when I go over this category?

Thank you for the input. I'm at a total loss!