Enter it here:

1.  Click Federal on the left-hand side menu.

2.  Click Income across the top.

3.  Scroll down to Investment Income and click Show more.

4.  Click Start or Revisit by Contracts and Straddles.

This, from TurboTaxMichaelDC:

Just enter Box 11 (gain or loss) in the appropriate TurboTax box.

To do this:

Under Investment Income, click Start/Update on "Contracts and Straddles"

Continue twice and check the box for Section 1256 contracts marked to market.

You will find that TurboTax offers only two boxes for Line 11 (gain or loss) and that is all that has to be entered.