Where do I enter contributions to a Cash Balance Plan?

If this is for an employer sponsored 401(k) plan, your contributions will be entered on your W-2 in box 12 with a code of D.  The contributions are not entered anywhere else on a tax return.

Where does a self-employed person enter the contribution to a Cash Balance plan?

Where to enter your SEP IRA contribution

https://ttlc.intuit.com/community/retirement/help/where-do-i-enter-my-sep-ira-contributions/00/26783

Where to enter Self Employed solo 401K plans

https://ttlc.intuit.com/questions/1901283-where-do-i-enter-my-401-k-or-thrift-savings-plan-tsp-contribution

You can contribute to a 401K up to the amount of your Net Profit reduced by 1/2 of the SE tax OR to the max contribution allowed for a 401k whichever is Less.

A cash balance plan is a type of defined benefit plan, so I suspect that it would need to be entered as a contribution to a defined benefit plan.