where to enter nys senior citizen exemption form it-201?

You don't need to enter anything on Form IT-201 to receive a tax benefit -- the TurboTax program will take care of that for you, if you qualify.

New York residents aged 59 1/2 or higher are entitled to an exclusion, of up to $20,000 of pension and annuity income, from their taxable income. If married, each taxpayer can qualify. 

TurboTax will apply the exclusion, if you qualify, based on the birthdate information you entered in the Personal Info section when you first started the program.

See Line 29: Pension and annuity income exclusion on page 9 of the Instructions for Form IT-201 for more information. If you can't open this file, it's available at Income tax full-year resident forms (current year).