Many resources can be exported from your content provider's application as a ThinCC or Thin Common Cartridge file format.
First, you'll want to find out from your content provider if there's a way to export your resources in ThinCC format. Files with the extension ".imscc" are either Common Cartridge or Thin Common Cartridge files, and Otus will be able to accept the Thin Common Cartridge version in the Bookshelf. Otus does not support full Common Cartridge files.
To add your resource to the Bookshelf, click the purple plus sign icon and choose Upload ThinCC.
Choose the ThinCC ".imscc" file from your finder window, any ThinCC files will show up in your Bookshelf for you to folder and organize. Full CC resources will not be added to the bookshelf, so you'll only find the ThinCC resources.
All resources uploaded from a ThinCC file will show up in the bookshelf with the icon below: