Where and how do I enter that my NYS pension is exempt from state tax?

After you enter the 1099-R, you will be prompted to indicate the New York pension exemption.  

New York special handling of pension distributions
TurboTax New York uses the following pension codes:

G - Qualified government or military pension
O - Optional retirement program distribution or former government pension rolled to an IRA
X - Not eligible for exclusion for those over age 59 1/2 (or their beneficiaries)
 

You can deduct certain federal, New York state, or local government pension distributions from your New York income. Government pensions include those from these sources:

US government, its territories or possessions, or the District of Columbia
Military
New York State, including State and City universities of New York and New York State Education Department (Optional Retirement Program)
Manhattan and Bronx Surface Transit Operating Authority
Long Island Railroad Company
Metropolitan Transit Authority (MTA) Police 20-Year Retirement Program
Local governments within New York
 

Those over age 59 1/2 can deduct up to $20,000 of certain IRA and other pension benefits on their New York return (if Married Filing Jointly, each spouse may deduct up to $20,000). Beneficiaries who receive pension benefits of deceased persons over age 59 1/2 may also qualify. We'll need to gather additional information when we work on your New York return.

New York doesn't tax Social Security or Railroad Retirement benefits. TurboTax automatically reduces your New York income for any benefits taxable on your federal return.