Hi Victoria,
I made a video to explain it to you :)
http://ferdykorpershoek.com/MegaMenu.mov
Kind Regards,
Ferdy Korpershoek
Hi again,
I kind of found a solution. It has to do with the custom height of my header area. It is 82 pixels. If I put it on large or slim the menu appears fine. So my next question is: How can I have a header with a custom height and still have the mega menu to appear normal?