I am assuming you are referring to the six-digit Identity Protection PIN. You will enter it in the Other Tax Situations section of TurboTax.
Log back into your account
Click either Take Me to My return or you may see an option to Fix your Rejected Return
Select Federal Taxes
Then, select the subtab of Other Tax Situations
Scroll down to the Other Return Info section
Click start next to Identity Protection Pin (see attached screenshot)
You can also use the search box to get to this section. Log into your account and click Take Me To My Return (must click this or will not get the jump to link in next step). Click the search icon in the upper right corner (magnifying glass) and enter "IP Pin" in the search box and click the "jump to IP Pin" link that appears in the search results. This will take you directly to the appropriate section.