First, navigate to that spot (I'll pick the 100 lb club) and copy the address in your browser address bar. You can highlight it and hit Control-C to copy.
Then, click the icon at the top of the area where you write new posts that looks like a globe with a paperclip...click on that. It'll open a window.
Then paste the URL into the window, and bam, you have a link.
http://www.3fatchicks.com/forum/forumdisplay.php?f=55
If you want to be fancy, you can write the name of the Forum/Thread and make that actual text the hyperlink. You do that by typing whatever you want the link text to be (I'll go with "100 lb Club Forum"), highlighting it, then clicking the link icon at the top of the posting area and pasting in your URL.
100 lb Club Forum