CalibreBox searches your Dropbox account for any files matching the Calibre library database filename. If your library does not show up in the library picker this is most probably due to the library not having been synchronised completely with your Dropbox or processed by the search feature. You just need to wait for the synchronisation to complete and wait for a short time after that for it to show up in the library picker.
When ypu add new books to your library and the library has been synchronised to Dropbox the app will show you a warning that the library copy on your mobile device is no longer up to date. You can update this local copy in the left-hand menu of the App Should your Calibre library not synchronise correctly try closing Calibre completely and wait for Dropbox to synchronise all changes in the background.
This is most probably caused by inclomplete synchronisation of your library to Dropbox.
If the App can open your library but fails to download books you should leave your computer running with an active internet connection for a while.
This ensured that Dropbox can upload all your books to its servers.
The download may also fail if the internet connection of your mobile device is insufficient.
Try downloading a book on Wifi to see if this is the problem.
If these steps do not work try reconnecting the App to your Dropbox account by using the "Change Dropbox account"-option in the left-hand menu of the App.
CalibreBox will work on any Android-Device from Android 4.0 (Ice Cream Sandwich) to 5.1 (Lollipop) that has the Google Play Store installed. The Google Play Store is requred due to the In-App Purchase option for the pro version.
