Get your taxes done using TurboTax

The answer posted above is from an older version of TurboTax and the steps listed have changed.   
 

You can choose to enter a 1099-B manually, but selecting the "I'll Type it in Myself" button on the bottom right of the import screen. 

From there, you will be asked to enter the broker/bank name and then decide whether to enter each transaction separately or in summary.  If choosing to enter separately, you can scroll down on the "Now we will enter your sale" screen and select "The cost basis is incorrect or missing on my 1099-B"  See a sample screenshot below.  

After checking that the cost basis is incorrect, continue through the screens until you come to one titled "We noticed there is an issue with your cost basis"   There you can select if you know the correct cost basis and want to make an adjustment, and then a box will open up for you to enter the correct cost basis amount.  See the second sample screenshot below.