If you want to find the fanfiction forum, simply scroll the main forums page all the way down before you hit the "Who's Online" table. You'd see forum sections such as Clubs and Games there. Right below RPG section, you should be able to find the Fanfiction forum. The main fanfiction forum is for Pokemon only. If you want to post non-pokemon fics, there is a sub-forum for it as well at the top.
Remember to read the rules there first before making a thread there. ^^