An employer miscalculated my state income taxrate on a w2. A letter was sent that I'm to enter my w2 as is and pay the under-withheld amount. Where do I enter the amount?

If not enough state tax was withheld, it will be added to your state tax liability (or decrease your state refund). 

You don't need to enter anything different, just enter your tax documents and let TurboTax do the calculations. 

Just enter the W2, if they "under-withheld' you'll square-up with your state return.

The "Under withheld amount " is in Box 17

There is no income tax on your W2.  The federal and state withholding are just estimates and prepayments towards the actual tax calculated on the income on your tax return.  Your employer (and you) have no way of knowing what your total income will be and what tax you owe.  You might have other income like interest and dividends or stock sales, etc.