The State of Illinois allows a subtraction from income for certain military pay earned, which is claimed on IL Schedule M Other Additions and Subtractions for Individuals.
Per Instructions for the Schedule M, on page 3:
Line 21 - 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:
For more information, see Publication 102, Illinois Filing Requirements for Military Personnel.
Attach Form W-2 showing your military pay. If you report dual status on Line 21, you must attach both Form W-2 and SF-50, Standard Form 50 (Notification of Personnel Action).
To enter this in TaxAct:
Note that any link in the information above is updated each year automatically and will take you to the most recent version of the document at the time it is accessed.