Try this: Click on the Federal Taxes tab, then Deductions & Credits, then scroll down to the various deduction headings on the lower half of the page (you may have to click "Skip to see all income" to display them).
Under the Retirement & Investments heading, select Traditional & Roth IRA Contributions and click Start/Update. Then follow the prompts.
TurboTax will ask you to check the box for the particular type of IRA you made the contributions to, and then it'll walk you thru the steps to enter your contribution, asking you various questions as you go.