Chances are that you have file folders on your computer hard drive that contain .pdfs of papers that you have downloaded in the past as you worked on various research projects. In order to populate ReadCube, simply drag and drop those folders of .pdfs into ReadCube. ReadCube will analyze the contents of those folders, identify the metadata for the files, and create citations for the journal articles. Journal article titles will appear as the file name for those .pdfs. For conference proceedings, you will see the file name as it appeared in your local folder.
To add a new .pdf to your library, select the import .pdf button, indicated below with the red arrow.
You can also import citation management libraries from EndNote, Mendeley, Zotero or another instance of ReadCube by clicking on the import papers icon and following the Other Library Import Instructions.