We can't offer support with CodeIgniter itself, as that's made by a third party, but we'll be happy to help with any questions or issues you have regarding the Dropbox API.
Apps should implement the OAuth app authorization flow to allow users to grant the app access to their account. If you haven't already done so, I recommend reviewing the Dropbox OAuth guide and documentation.
If you're still having any trouble please reply here with the details, e.g., the relevant code an any unexpected error or output you get.