Once you click "Post Reply" to submit your post you will see three 'buttons' appear on the bottom left portion of your post underneath your signature (if you have one) - EDIT, DELETE and REPORT.
Like Jen already said, you only have 15 minutes to edit your post. After that, the EDIT 'button' will disappear and you will no longer be able to edit your post. This was done to help battle spammers that visit the forum.
Well, that explains it! I knew I had seen an edit button. But this morning I was trying to edit something from a couple of days ago. Thanks, Jen and Chef for answering my question!