You can click in the box right after the "1", hit your spacebar, and type whatever you like, so that it reads 1 oatcake, 1 cake, 1 piece, etc. You can even delete the 1 and replace it with another number, if you consider that to be what you want to call a serving.

On the front of the label, there should be a Net Weight printed somewhere. That's the weight of the food inside the package, minus the weight of the packaging. So, if it says "Net Wt. 200 grams" and there are, say, 4 oatcakes in the package, each one must weigh 50 grams. (If it gives the weight in ounces, about 28 grams are equal to one ounce.) Just delete the 100 in the "grams" box on the Custom Food form and replace it with 50 ... or whatever number is appropriate for what you have set as a serving size.

HTH!