Get your taxes done using TurboTax

When you report a sale of stock, you will be asked if it involves employee stock, to which you will answer "yes":

When you enter the sales information, you will pick Incentive stock option for the type of investment you sold and check the box that says the cost basis is wrong or missing:

On the next screen you can indicate that you need help finding the cost basis, and then you will see the screen where you can enter the information off of your Form 3921 to determine your correct cost basis to enter for the stock sale.