You'll enter this information by clicking on the Federal Taxes tab, then select Other Tax Situations, then scroll down to Additional Tax Payments and click on Underpayment Penalties. Then just follow the prompts.
TurboTax will ask you a variety of questions (some may apply, some not), and eventually you'll be prompted to enter your 2015 tax liability for 2210 purposes (Form 2210 is the underpayment penalty form) and 2015 AGI.
That may take care of the penalty. But if you had 2015 AGI over $150,000, then the amount of 2016 withholding & estimated tax payments would have to exceed 110% of the tax shown on the 2015 return, rather than 100%.
