At the top of your screen, there should be a header named 'my mibba'.
Click on it, and it should excort you to your 'editing page'.
Once your there you should see, on the right side of your screen there is a bunch of orange headers with 'messages', 'account', 'profile', etc on it.
look under the one that says 'stories'.
Then click the link named 'my stories'.
After that it will say 'add new story'. Once you've click that...You should be able to post up your fic.