You don't enter it anywhere on your tax return because they are already reflected on your W-2. All you have to do is report W2 data in Turbo Tax exactly as it appears on the form.
The 457 plan contributions you elect to make come directly out of your salary. Since the contributions are made with pre-tax dollars, your employer does not include these amounts in your taxable income for the year. At the end of the year, when you receive your W-2 form that shows your earnings, you will notice that your wages subject to federal income (box 1) tax are lower because of your 457 plan contributions (box 12). Since the contributions are not counted in your taxable income to begin with, you do not take a deduction when you file your return.
Note: If a plan allows after-tax contributions, they are not excluded from income and an employee cannot deduct them on his or her tax return - per IRS.