Illinois state return, where do I enter my 529 contribution?

When you go through the IL state interview, look for the screen titled Here's the income that Illinois handles differently.   Scroll down to Education.   See the options below:

College Savings - If you added money to a college savings pool or pre-paid tuition program, this applies to you.  This includes Bright Start and Bright Directions college savings pools and College Illinois pre-paid tuition programs.

Illinois ABLE - With Illinois ABLE, you can save for qualified disability expenses.  The earnings on your investments are federally tax-deferred and tax-free, if used for qualified disability expense.   If you added money to an Illinois ABLE program, this applies to you.