As you are preparing a Missouri Nonresident return, you will be asked about various deductions, etc that are specific to Missouri.
You will come to Income Allocations and Other Income Allocations. The total income from all sources will be shown and you are to enter the amount of income that is due to Missouri sources. Then you will be asked about Federal Adjustments (which includes Missouri Additions and Subtractions).
This will allow you to make sure your income is not considered to be taxable income on the Missouri return.