I am not with Laminar Research; just a flight simmer.
The button you are looking for is a little obscure. After your initial login to the forum click on the "ask a question" or "answer" or "comment" black box icon. A box will appear with the heading "Your Answer" etc
At the top of this box there two rows of icons. In the middle of the second row there is an icon that looks like a chain link (to the right of the capital A). Click this icon and another box will appear.
The heading on this box will be "Link". You will see three tabs. Click on the tab labelled "Upload". Another box will appear. Click on the box titled "Chose file" and this will take you to your file directory structure. Find the file you wish to attach. Highlight the file with a "right mouse click" once making sure the file name is in the "File Box" and then click "open"
You should now see the file name beside the "Chose File" box. Then click the "Send to Server" box. Then click the green "OK" button.
Your comment (answer or question) should re-appear with the file (log.txt) attached and highlighted (blue). If attached correctly the file will download onto any PC to view.
Good luck. Hope it works for you.