State specific modifications have their own input screens.  These are generally found under State & Local > Modifications > XX Modifications.  In this case, it'd be WI Modifications.

If you scroll down the input screen, you'd find a section called Wisconsin Schedule I Adjustments.  Enter the amount taxable to WI on the line for Certain student loan forgiveness.  This will then flow to both Wisconsin Schedule I, line 1g and line 2 of Form 1.

I am able to get to the WI Modifications but the input page does not show anything related to schedule I.  There is a list of input boxes, but none of them reference student loan forgiveness.  

Not sure what I am missing, but the input page does not look like how you describe it to be.

Are you using ProConnect Tax Online?

Can I ask why it is not reported on their Federal tax return? 

Schedule I for WI, Part 1 line 1g