Kate:
First go to your 'my mibba' page.
Then scroll down to the orange box that says 'Stories.'
Then click the blue box that says 'Stories'
It will be on the right hand side.
Then you will be on a page that has a list of your stories.
Click the title of your story.
Then you'll come to a page that will have a list of the chapters.
There should be a 'Add Chapter' link in blue under the list.