Have you already filed, or is your return still in preparation? The following info assumes you are still in the preparation stage and have not yet filed:
Are you able to return to the My Info section OK as @xmasbaby0 mentioned above to add a dependent? Or does TurboTax think you were finished and has closed your interview? If the latter, you have to open the unfiled return back up to make any edits. To do that, at the Tax Home look for a link "Add a State." That doesn't really add one, but opens the interview back up.
Again, don't use that method if you have already filed, and it's been accepted. If you already filed, report back for more information.