On the Food List tab there is a conversion tool at the top. Use the left side to do simple conversions between G, Oz, Tbsp, and Lbs. Be sure to adjust the nutrtiaionl values (Protein, Carbs, Fat, Kcal, etc...) to match your units and quanitity.
The right side is if you want to convert to anything else, assuming you know how much of what you're looking to convert to (indicated as "x") is in 1 unit. This is useful when paired with the Food Database since all of the foods are measured in 100g so that you can still use those nutritional values. For example, lets say you want to convert the nutritional values for 100g of raw blueberries into the unit "cups." You would click the "look up food online" button and determine how many grams 1 cup ("x") of blueberries weighs (or weigh it yourself), which comes to 148g in 1 cup. So by putting that into the top right, you'll know that there is .676 cups in 100g for this food and be able to use the nutritional values from the Food Database.
When in doubt or if you want to enter nutritional values specific to a particular brand of food, you can click the "Look-up by Brand" button at the top of the Food List tab and manually enter in your values.