I have a 401K through my employment and I would like to contribute a catch up amount . Where do I enter in my return?

[dmertz]

Assuming that you are self-employed, TurboTax automatically includes your catch-up contribution when it calculates your maximum permissible self-employed retirement deduction for 401(k) contributions.  If you are contributing less than the maximum, enter the catch-up amount in the Catch-up Contributions box on the Individual and Roth 401(k) Plans page.  You only need to enter an amount as catch-up once you have reached the $18,000 limit for regular elective deferrals.

If you are referring to a catch-up contribution at an employer that is not yourself as self-employed, that contribution was required to have been made through deferrals from your salary paid during 2016, you can't add it now for 2016.