Where do I enter mortgage interest?

Mortgage interest for what? Rental property? Primary Residence? 2nd home?
For your primary residence or 2nd home, you enter it in the Your HOme section under the deductions and credits tab.
For rental property, you enter it on the screen that asks for it as you work through the Rental and Royalty Income & Expenses (SCH E) section of the program under the Personal Income tab.

Primary home (my residence).  It's grayed out so I can't enter mortgage interest.

That's because at the beginning in the initial interview, you told the program that you did *not* own a home in 2016. Go back to the personal info tab and start over. Change only what needs to be changed.

Go to Federal> Deductions and Credits> Your Home to enter mortgage interest, property taxes, private mortgage insurance and loan origination fees (“points”) that you paid in 2016.  You should have a 1098 from your mortgage lender that shows this information.