I'm not sure how the free online edition works, but when you do the federal review it should start with a question about whether you received stimulus payments. Answer 'yes,' and it shows the amounts of each you were eligible for and asks if you received them. If you never received the second one, answer 'no' and it should then add the $600 amount to your refund. Look at line 30, recovery rebate credit, on your Form 1040, and it should have the $600 credit there. If you owed rather than having a refund, the $600 would be applied to that, reducing what you owe.
