On the page that lists the Forms 1099-R that you have entered, click the Continue button. TurboTax will then ask you to provide your Roth IRA contribution basis. After you have entered that, TurboTax will be able to complete Part III of Form 8606 to correctly calculate the taxable amount to be included on Form 1040 line 4b (which will be zero if you took out no more than your contributions).
