You can go to your game archive at http://www.chess.com/home/my_archive.html?show=live where you can then click to view one of your live chess games and then you should see a link towards the top right side of the page which you can click for "Computer Analysis".
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In the FAQ, it says "After you complete a chess game in the Live Chess or Online Chess play you can click on the "Computer Analysis" link on the game page to send the game in for the computer programs at Chess.com to analyze."
There is never a "Computer Analysis" Link for me to click.
How do I get my game analysed?