Go into Setup -> Category Setup (or click the Category Setup icon at the top of the category list or use the Setup menu). In the Category Setup, enter numbers as monthly budgets. You can set budgets for the specific month you are browsing, or set normal budgets for that category.
If a budget for a specific month is set, it will override the default budget entry. Once you enter these values and save the Category Setup, the figures on the category sidebar will automatically update, which are the difference between your actual spending and your budget plan.
Note: If you leave a category budget blank, it defaults to $0, and thus the sidebar figures are simply a negative total of all of your spending ($0.00 – actual spending = result).