Upon completion of a game, you can click on Computer Analysis. After some time you will get a Chess.com message saying it is complete. When you go to the game, you can see how the computer thinks you played, what moves it didn't like, and what it thinks yu should have done instead.
Edit: The Computer Analysis button is on your game view once the game is done. You can view any past game and select it.
I've seen a demo for that. If you are rated below 1800, I would not even look at the inaccuracies. It will go 6 moves deep and say "and now White is slightly better". I clicked on that postion and could not see the advantage. It does catch blunders well, though. In the demo game, there were 4 blunders made by one side, including not catching the opponent's blunder. I saw two of them that were obvious. One of them was that player attacked the king instead of capturing a piece. The attack could have been refuted with a very good move, but the opponent did not make that move, and was checkmated. So sometimes a blunder is not such a blunder against a weaker player.
I bet some of my games would have 10 blunders. I wonder what percentage of my moves would be inaccurancies. 50%? 75%?
Nothing can make you feel worse than a computer analysis 
Don't do it on games you think you did well on if you're low on confidence, it'll make you look like an idiot. The number of games I've won where I thought I did some brilliant tactical thinking were shot down in flames by even the 2000 computer analysis.
You need to go to either your online games history or your live chess games history to find a game you want the analysis done for.
Your online games are shown on the bottom of your current online games page
http://www.chess.com/echess/myhome.html
Your live chess games are shown on the bottom of the page where you go to enter live chess.
http://www.chess.com/livechess/myhome.html
Click "view" next to the game you want analysis done on, then the game opens. Look to the right, above the small notations box and it says "Games Analysis". Click on that.
Once it says "processing" you can then leave that page. Once the games analysis is done, it will be sent to your inbox emails.
If you need help, please contact our Help and Support team.
I just recently upgraded to a Diamond account and I was wondering how to use the computer analyisis as it is in the demo. With a computer pointing out blunders and correcting your mistakes.