If you are using TurboTax Online Self-Employed, you can find the self-employed retirement contributions section by going to Federal > Income and Expenses and then scrolling down to the Other Business Situations section. Click Start or Update beside Self-Employed Retirement Plans.
If you are using TurboTax CD/downloaded Home and Business version, start by going to the Business tab at the top of the screen. Then, scroll down to Less Common Business Situations and click Start or Update beside Self-Employed Retirement.
For more information, see the following TurboTax article:
Where do I enter my SEP IRA contributions?