When creating navigation menu for your store, very often such necessity happens as to add any product category as the item or a sub item. This is very easy to implement. Go to Appearance > Menu. In the top of your window you should find “Screen Options”. Click on it, a panel will slide open with lots of check boxes.
Choose the right you need. Then you can see two check boxes labeled Categories. One of them – is you Posts categories, another – Portfolio categories. It will be enough clear for you. But for now we need Product Categories.
After that your Product Categories will appear in the left sidebar. And now you may add them as an items.
Save the menu and you will see appeared items-categories on your frontend.