Having trouble adding drop down to my header menu

I'm using Layers Pro on my site (www.artistsalley.net). I would like to add a drop down menu for my product categories. I've tried following the instructions in this tutorial: https://premium.wpmudev.org/blog/daily-tip-how-to-display-all-your-wordpress-categories-in-a-drop-down-menu/
but I can't figure out where to put that code. I have a global product categories, but I can't figure out how to insert a drop down in the header area. I've indicated where I'd like it to go with the black arrow in the picture. Can you help?