We’re in the process of updating the documentation for Zotero 5.0. Some documentation may be outdated in the meantime. Thanks for your understanding.
How do I manually add a bibliographic item?
You can manually add items by clicking the green circle with a plus icon above the middle column, selecting the appropriate item type, and filling in the bibliographic details in the right column. New items will appear in the “My Library” folder and can be organized into other folders.