Once you've logged in and opened up your tax return in progress, click on Federal Taxes, then Deductions & Credits, then scroll down toward the bottom of the page (you may have to click "Skip to see all tax breaks" to display all the various deductions) and click on Employment Expenses. Next to Job Expenses For W-2 Income click the Start/Revisit button. Then TurboTax will show you a couple of screens with various expenses listed, and you'll have to check if any of those expenses apply to you. You'll see Union Dues listed on the 2nd screen, so check "yes" there.
Continue thru the various screens and answer the questions (many of these items may not apply to you), then you'll come to a screen that says Job-Related Expenses and you'll see two columns for writing in expenses: Description and Amount. This is where you'll write in Union Dues and the amount you paid. Then continue to the end of the section and hit Done.
