After-tax 401(k) contributions are reported nowhere on your individual tax return (except in the case where those contributions qualify you for a higher Retirement Savings Contributions Credit than you might already be eligible to receive, but it would be extremely unlikely that anyone making after-tax 401(k) contributions would qualify for this credit at all).
You employer includes catch-up 401(k) elective deferrals in the amount reported with code D in box 12 of your W-2. Enter amounts shown in box 12 of your W-2 only in box 12 of TurboTax's W-2 form, nowhere else in TurboTax.
