How do I enter my 1099-DIV from Thrift Savings Plan?

Go to the Income and Expenses section in the Federal area of TurboTax. Choose the Interest and Dividends option, then Dividends on 1099-DIV. Start that section and answer the questions to enter your form 1099-DIV.

Seems unlikely that you would ever receive a 1099-DIV from the federal TSP.  Are you sure it's not a Form 1099-R?

Good catch.  I got a 1099-R from TSP.  Thanks