Medicare Part C payments are the payments that you yourself make to the private insurance company that is providing your Medicare Advantage plan. They do not come out of your Social Security benefits. You would enter those separately as deductible medical expenses for Schedule A as you would any insurance premiums. Some Medicare Advantage plans have $0 premiums, so in that case there would be no payments to deduct.
(If you are self-employed you could instead report them as self-employed health insurance, but since you indicated that you are using the online Free edition of TurboTax, you are apparently not self-employed.)
