Retirement tax questions

Yes, you must enter your W-2 as-is.  When Turbotax sees the box 10 amount, it will ask you "did you forfeit or carry over any amount?"  (That question might be in the W-2 interview or the Dependent Care expense interview, I forget.)

If you carried over or forfeited any amount of the box 10 dependent care benefit, it won't be added to your taxable income.  But if you were able to withdraw the money, then it will be added back to your taxable income unless you used it for care of a qualifying person.