Most forums have a limitation as to how much time after posting you can edit your thread/comments. This is in place so you don't just edit out information when you wish and possibly mislead other users. I believe the standard in most places is 15 minutes after the post. After that, the button disappears.
If you don't see an Edit button under your post, chances are that time has passed. So what you need to do is to hit the Report Post button (on top of the post/thread) and explain why you need to edit the post. The moderators will probably take a little time to check it out and if all is ok - they will edit it for you.
Hope that helps