Look at the main section page showing all of the threads and other stories people have posted (http://www.serebiiforums.com/forumdisplay.php?f=32), and then click on the 'new thread' button which is above the sticky threads at the top, or at the bottom of the page.

Then add in the title of the story in 'Title', copy and paste the story into the text box, preview and clean up the presentation if you want/need to, and then hit submit thread.