Where do I enter contributions I made to NYS 529 plan on my State return? I have completed these returns and haven't been prompted anywhere to enter this.

The input for the 529 plan deduction is in the state interview questions under the screen titled "Changes to Federal ( Please see the attached photo). 

Once you are on the page, please look for Education adjustments, then New York 529 college savings account activity, click Start. Then you will be able to enter your contributions in the next screen, Enter New York 529 Contributions

Those boxes are not in the NY return entry screens.

The instructions provided by @NicoleU apply to both TurboTax Online and TurboTax CD/Download @gpaultax.

They are not on the New York tax form. They are part of the TurboTax interview.