If you did a direct rollover from your pension plan to the 401K, your 1099R should have "G" in box 7. That code indicates it is a non-taxable rollover to the 401K. If the coding on the 1099R form from the pension is not a "G", after you enter the 1099R in TurboTax, you will see a series of screens. When you reach the screen "Did you rollover the gross amount to another retirement account?" enter "Yes"
A distribution reported on a code G Form 1099-R is only non-taxable if there is no nonzero amount shown in box 2a of the form provided by the payer. A rollover from a pension buyout to a traditional 401(k) account should have a zero in box 2a.