The Sec125 amount is already deducted from the wages in box 1 of your W-2. The employer subtracts it when they calculate the amount of wages to report. You can't deduct it again on your tax return.
If you live in New Jersey, and the state wages in box 16 of your W-2 is higher than the amount in box 1, you might be able to include the Sec125 amount in the medical expenses on your state tax return. I don't know if any other states have the same arrangement. Check the rules for your state.