You should be able to click on the little brown chess board icon (the chess diagram editor) on the left side of the tool bar right above where you type your post.
For more information on this tool please take a look at http://www.chess.com/article/view/how-to-use-the-chesscom-diagr
Regarding the questioned problem, a lot of times in games tactics arise due to miscalculations on the part of your opponent. In that problem, it shows an example of that where the opponent didn't realize that his bishop was pinned and that the queen move just dropped his queen. Just because it is a "computer move" by no means suggests that a computer engine makes the move and it is the best move. Most of the time, the "computer" move is a blunder that you need to capitalize on to get the tactic correct.
Can anyone help? I am trying to insert a chess diagram here. I have been using tactics trainer and I am having a hard time understanding the computer moves at times. In this problem (0043246) the computer seems to sacrifice its queen for no reason. If anyone knows how I can insert this diagram in a forum topic I would appreciate the help so people could discuss the problem. Thanks. I have tried using the posting tips to the right as I write this post but it doesn't seem to work.