Add categories in page menu bar

Hallo all!

I've bene enjoying the Atahualpa Theme!
I'm making a more usable page menu bar now.

So far i have this: http://coachpraktijk.be

I want to include a menu item 'Artikels' (i have already created a page for this) that gives a dropdown menu taht lists my different post categories.

How to do this with coding ? (i'm hesitant on using yet another plugin since i want to keep site performance)

Thanks!
Fred