The State of Illinois allows a subtraction from income for certain military pay earned, which is claimed on Schedule M.

Per the Instructions for the Schedule M, page 3:

Line 20 - Military pay earned

Enter the amount of military pay that you received from the U.S. Armed Forces or the National Guard of any state, or that you received as a dual status military technician.

You may not subtract the following:

1. Combat pay that is not included in your federal adjusted gross income, Form IL-1040, Line 1
2. Pay you received
   1. Under the Voluntary Separation Incentive,
   2. From the military as a civilian,
   3. Under the Ready Reserve Mobilization Income Insurance Program,
   4. For duty as an officer in the Public Health Service

For more information, see Publication 102 Illinois Filing Requirements for Military Personnel.

Attach Form W-2 showing your military pay.

To enter this in TaxAct:

1. From within your TaxAct return (Online or Desktop) click State and then click Illinois in the expanded State menu.  
2. Expand Income and click Subtractions
3. The program will proceed with the interview questions for you to enter or review the appropriate information
4. Answer Yes to the question: Would you like to enter the other subtractions from income Illinois Schedule M?
5. Click Continue once
6. Enter the amount in the field Military Pay Earned at the bottom of the screen.