If you search for a food that isn't in the database, an "Add New Food" option will appear at the bottom left of the listings box. Click on that and it'll let you put in the details of the new item.
You can also edit the details after you've saved the food item, by going to the summary box that appears on the left of the screen, when you go to enter it in your food diary.
"There's no secret. You train a lot and eat right" (Mo Farah, British Olympic marathon runner).