How to add a subcategory in the menu?

Question

I have menu with category: root1, root2...
And root 1 have subcategory child1 and child2. And i want add new subcategory child 3 in child1.
And when you click on root1 to open all subcategories. Please check and modifity file. Need use linqto sql.  I try, I did not get.
Please, help