What is the deduction that you are trying to make? There are a few different things that can go on Line 25600.

Please answer OP's question. The deduction is irrelevant. 

To claim other deductions on line 25600:

Click on other deductions on the left side menu

Click continue on all the pages until you reach the "Additional deductions" page as seen in the following screenshots:

TurboTax Online

TurboTax Download/Desktop 

I hope this was helpful

I have clicked through the entire deductions screen, maybe blind but never came across 'Additional deductions' options.  Not available in the find as well.  Intuit, if I can't enter a simple line from an established tax form, my confidence in your product is sorely tested.  Perhaps the ability to enter line 25600 is outside of the parameters of the product - if so, why was this not specified in the purchase sheet.  Finally, why on earth can I not simply go to the forms and enter into 25600 on the online version?