When you are in the Food section and hit the + button at the top right, on the next page you'll see Frequent, Recent and Custom at the top. Select Custom and there'll be an option to Add Custom Food at the top in red text.
It can be quite the challenge getting the correct calorie info, as I see some wildly different numbers on different websites, but I go with something close to where the majority think it should be.
Hope this helps.
