Did you carry forward your return from last year? If so, your losses will automatically populate.
Click the "Deductions" tab and hover over 'losses'
select "Net Capital Loss Carry Forwards". Make sure the losses are showing for previous years. click 'continue'
The next page "Net Capital Losses Being Claimed" will show you how much gain you have in 2017 and you can enter how much of the loss you would like to claim up to the amount of the gain.