In the subject line, you say IRA, but the body of your question says 401(k). These are different plans.
If this is a 401(k) plan offered through your employer, it is already listed on your W-2 and will be recorded when you enter that form. If it's an IRA, go to Federal > Deductions and Credits. Only Traditional IRA contributions can be deducted; Roth IRA ones cannot.
