I'd love for this, too. I just had a low-carb energy drink, and I can't find it anywhere on the food list. I'd love to be able to add it, so I can keep better track of things.
It's obvious that multiple people added to the list at one point in time, as there are multiple entries for an egg mcmuffin and things like that. But WHERE do they get added? And how?
Whenever you search for an item in the food list, it has an option to "Add new food" at the bottom of the grey border (or at least it should, if everything's working correctly.) Simply click on that and fill in all the nutritional info, then save it to your food list.