Where to post contributions to college fund 529 on SR1040?

For most people, you don't enter it, at all.  There is no federal deduction for making a contribution to a 529 plan, so there is no place in the TurboTax (TT) Federal program to enter it.  TT will not be tracking your contributions.

 Some states allow a deduction on the state return. If your state is one, it will come up in the state interview; usually in the "Here's how (your state) handles income differently" section.  If you state allows a carryforward, the TT state program will track that.

 For a state list of eligible plans, see: https://www.savingforcollege.com/article/how-much-is-your-state-s-529-plan-tax-deduction-really-wort...

If you state allows a credit for the 529 contributions, it would be deductible on your state tax return.  This in not a credit on your federal tax return.  

If your state allows the deductions, you will be prompted in the interview process of your state return.  

@coarias