Oh shoot! You should be able to go through your own Dashboard and find it and edit it to add chapters?

Check the left hand side of your AO3 dashboard and choose "works" or maybe "drafts"? If that doesn't work, try going into the HBB Collection and finding your fic - even if they're all listed as "mystery works", if you're logged in and yours has been submitted, you should be able to find yours highlighted in grey and able to click in and work with it.

Let us know if you're still having problems?