I have a 1099-B from a broker for futures but it gives only net gain or loss, with no itemized bases or proceeds. turbotax only allows to enter basis and proceeds.

That is correct.

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.